Psa 138:2 I will bow down toward Your holy temple And give thanks to Your name for Your lovingkindness and Your truth; For You have magnified Your word according to all Your name.
I believe that we often just look to see where in the Holy Scripture we see the word, "word" and believe that the word must mean "scripture" but in reality even the words of the Holy Scripture testify* about Christ who IS "the" Word.
Fundamental.
Jesus also said that those who would follow his teaching will follow the teaching of the Apostles. Now, the reason the Apostles epistles are considered authoritative to begin with is because the Apostles were considered authoritative. They were not authoritative only insofar as they wrote letters otherwise we would read in the Holy Scriptures them praising those who "kept" what they had heard from them and learned from them face to face (even before the epistle arrived to the Church addressed)
So I will caution you to understand that when it comes to "authority" and the word "word" the scriptures themselves, though they are authoritative, this authority also rests in "sound teaching" or "doctrine". And the way that the early Christians helped to discern between the two was not by scripture alone* but rather by keeping to the pattern of sound teaching which those early communities had received from the Apostles themselves.
That's why the Holy Scriptures point toward the teaching of the Apostles that were given to those communities prior to the epistles (letters) even being written.
This is why we read, "keep to the tradition which you have heard* from us".
The IS Holy Tradition and the Holy Scriptures themselves point toward it.
2 Tim 3:16-7 All Scripture is inspired by God and profitable for teaching, for reproof, for correction, for training in righteousness; so that the man of God may be adequate, equipped for every good work.
Consider the following please and see if the early Church didn't see things outside the epistles they received as also being authoritative.
Luke 10:
16"He who listens to you listens to me; he who rejects you rejects me; but he who rejects me rejects him who sent me."
Is the above speaking of the Holy Scriptures?
1 John 4:6
We are from God, and whoever knows God listens to us; but whoever is not from God does not listen to us. This is how we recognize the Spirit of truth and the spirit of falsehood.
Do you see how the early Christians used their time with the Apostles face to face in order to discern not only false teachings from true teachings but also in regards to how to approach scripture. Does the above say that how we recognize the Spirit of Truth is merely to read and come to your own understanding of the Holy Scriptures or that it is directly related to the faith as it was handed, in the manner it was handed, to those communities by the Apostles.
2 Corinthians 10:8
For even if I boast somewhat freely about the authority the Lord gave us for building you up rather than pulling you down, I will not be ashamed of it.
Were the Apostles given Authority because they read the right meaning into Holy Scripture but because they themselves were given Authority by Christ? It is not their own Authority but the Authority given to them by God. What they taught, though they are men, are not commandments of men, but God, because they were "sent". Did the early Christians consider the Apostles to be authoritative? Sure they searched the Scriptures to test if what they said was true but again! when it says "scripture" in the New Testament, know! that it was speaking of the Old Testament.
In other words, it was not unlike St. Paul was saying, test the authority of the New Testament by reading the Old Testament and also, see if what we say as to what those epistles "mean to say" matches with what the Old Testament means to say haven been given the revelation of Christ through (in most cases) the Apostles. Test to see if what we orally transmitted to you is true in the Old Testament as well.
1 Thessalonians 4:2
For you know what instructions we gave you by the authority of the Lord Jesus.
If you had met the Apostles face to face and they wrote you such a letter as Thessalonians, not only! would you see the letter itself as authoritative but the preaching and teaching they had done in your very presence. And as the letter to the Thessalonians itself says, "by the authority of the Lord". Both the oral deposit of faith into the communities given by the Apostles AND their letters and gospels were authoritative.
Hebrews 13:17
Obey your leaders and submit to their authority. They keep watch over you as men who must give an account. Obey them so that their work will be a joy, not a burden, for that would be of no advantage to you.
If the scriptures alone! (sola scriptura) have authority then the leaders can not have authority. But here Hebrews says itself that the leaders appointed by the Apostles and those the Apostles appointed did in fact HAVE authority.
Titus 2:15
These, then, are the things you should teach. Encourage and rebuke with all authority. Do not let anyone despise you.
See too that the letter are authoritative but recognize that authority resides not with the scriptures alone. It can not and have the bible mean what it means to say.
2 Thessalonians 3:14
If anyone does not obey our instruction in this letter, take special note of him. Do not associate with him, in order that he may feel ashamed.
Now again, the letters are authoritative and the ancient faiths agree on this too. However, we have to admit that the reason this second letter to the Thessalonians was authoritative EVEN BEFORE they even read the letter is because it was given to them by the Apostles of whom God sent. So if the Apostles taught you for instance the right teaching on baptism etc. then you would use that experience as a community having met the Apostles face to face to help inform your reading of the text.
Also notice above that this is hardly a verse that encourages us to embrace pluralism. If you have a system that embraces lots of contradictory teachings (think of Baptists, Anglicans, Pentecostals on the laying on of hands) and yet you "associate" with them as being in "communion" with them then how on earth are we to reconcile this reality with the commands given to us by the Holy Epistles?
Romans 15:18
I will not venture to speak of anything except what Christ has accomplished through me in leading the Gentiles to obey God by what I have said and done—
Notice that is say "by what I have said and done". Not only or even "what I wrote". If those imitated and kept what they heard and saw St. Paul say verbally and do physically then they preserved something that can not merely be had by scripture but through imitation. We see in the bible that we are to preserve such teachings not only in word and written word but also in doing. Doing helps put much in context because if we can't do as it says or differ on how to "do" a thing said, then the early Christian communities only had to rely on their personal experience with the Apostles and remember what they had "done" in their very presence.
Acts 16:4
As they traveled from town to town, they delivered the decisions reached by the apostles and elders in Jerusalem for the people to obey.
How is it that elders have authority to give decisions for the people to obey? The Apostles AND the elders. Did the elders lose this authority when the Apostles died? Where in Holy Scripture does it say that the elders will lose authority after the Apostles die? Did all early Christian communities die out only to be resurrected in the late 15th century?
Not even the 15th century protestants believe as the modern day protestants. Are we to believe that this "authority" or "church of whose foundation is truth" had not yet emerged until today? Perhaps today is not even the day and there is no church right now. perhaps we will have no church until tomorrow? Where in Holy Scripture do we affirm such beliefs?
John 15:20
Remember the words I spoke to you: 'No servant is greater than his master.' If they persecuted me, they will persecute you also. If they obeyed my teaching, they will obey yours also.
This was said to the Apostles. Jesus, Our Lord, did not say that if they obeyed my teaching then they will obey yours insofar as the letters you write go but nothing outside of that....
John 14:23
Jesus replied, "If anyone loves me, he will obey my teaching. My Father will love him, and we will come to him and make our home with him.
Read John 14 and 15 in conjunction and you see that there is a marriage of sorts that aligns itself to the "whole" of Holy Scripture.
Hebrews 13:9
Do not be carried away by all kinds of strange teachings.
If a mere two teachings are contradictory then at very least one is wrong. Yet we embrace all these teachings which are untrue? How strange. How do we reconcile this practice in Holy Scripture?
Hebrews 6:1
Therefore let us leave the elementary teachings about Christ and go on to maturity, not laying again the foundation
Read Hebrews 5-6 and see if you can find any two denominations that agree on all the elementary teachings listed within Protestantism. The fact that so many contradict each other on just the elementary teachings alone is a sobering commentary on the state of the pluralistic nature of modern protestantism.
Titus 1:11
They must be silenced, because they are ruining whole households by teaching things they ought not to teach
Two contradictory teachings can not be right yet both base their interpretations on the authority of scripture. Yet, one is or both are wrong. How do we reconcile this if it doesn't even match the requirements set forth by the Holy Scriptures?
2 Timothy 4:3
For the time will come when men will not put up with sound doctrine. Instead, to suit their own desires, they will gather around them a great number of teachers to say what their itching ears want to hear.
How can we tell what is sound doctrine? Well a protestant will say to just read the bible but then you get as many interpretations of what the scripture says as there are readers. How did the early Christians do it?
The answer follows:
2 Timothy 1:13
What you heard from me, keep as the pattern of sound teaching, with faith and love in Christ Jesus.
What you heard from me. Keep as the pattern of sound teaching. Use that which you heard to help you discern.
This is what the more ancient faiths talk about when it comes to Holy Tradition.
2 Timothy 2:17
Their teaching will spread like gangrene. Among them are Hymenaeus and Philetus
We are called to preserve the teaching of the Apostles not merely the outward authority of the scriptures used to justify a multitude of ever growing ever mutating contradictory teachings.
2 Timothy 3:10
[ Paul's Charge to Timothy ] You, however, know all about my teaching
How did he know all about his teaching? Had he yet read 2 Timothy and yet already knew all about his teaching? He heard it. He saw it. He experienced it. He imitated it.
2 Timothy 2:2
And the things you have heard me say in the presence of many witnesses entrust to reliable men who will also be qualified to teach others.
He passed what he has experienced on to others who would then be entrusted to preserve those teachings as well. Not merely to preserve the letter (we know St. Paul wrote a letter that was lost and yet nothing less is known). This is the tradition that the more ancient faiths speak of as being Authoritative.
1 Timothy 4:11
Command and teach these things.
Notice that it doesn't say, "what does this verse mean to you, teach that" or "in my opinion I think that the bible is trying to say x,y,z."
1 Timothy 1:3
As I urged you when I went into Macedonia, stay there in Ephesus so that you may command certain men not to teach false doctrines any longer
When you have a huge group of contradictory teachings and everyone embraces each others right to do so how does this differ from the early Church. It is said to 'comman" that certain men not teach false doctrine. Yet, you embrace the contradiction within the whole of a certain system but not outside of it? Reconcile this in scripture if you can or at least consider the role of Holy Tradition.
2 Thessalonians 3:6
In the name of the Lord Jesus Christ, we command you, brothers, to keep away from every brother who is idle and does not live according to the tradition you received from us.
Did you receive this tradition from the Apostles? Luther, Zwingli, Calvin, the guy down the block who just started his own flavor? Do you fulfill the scriptures and see them as authoritative when we are told not be in communion with them or do you embrace this pluralism? How is this in line even within sola scriptura? Are these the teachings of men?
1 Corinthians 4:17
For this reason I am sending to you Timothy, my son whom I love, who is faithful in the Lord. He will remind you of my way of life in Christ Jesus, which agrees with what I teach everywhere in every church.
did Timothy remind them of the Holy Scriptures alone or was it St. Paul's way of life in Christ which AGREES with what is taught everywhere and in accordance with ALL the Apostles.
Holy. Tradition.
Research those early Christian communities that ran into the Apostles and see what they had in common.
Acts 18:11
So Paul stayed for a year and a half, teaching them the word of God.
That's a lot of instruction. That's a lot of fodder to help that community know how to behave in the house of God. That's a lot of experience that can be used to help them know what the bible "means to say". Takes a lot of guess work out of it.
Acts 2:42
They devoted themselves to the apostles' teaching and to the fellowship, to the breaking of bread and to prayer.
The Apostles teaching. Does it say merely "Holy Scripture" alone? How is it that we have a different standard apart from our early brothers and sisters? Shouldn't we have the same devotion?
2 Corinthians 11:4
4For if someone comes to you and preaches a Jesus other than the Jesus we preached, or if you receive a different spirit from the one you received, or a different gospel from the one you accepted, you put up with it easily enough.
Do we put up with it easily enough in a system where there are more opinions on what the scriptures mean to say than there are people? Do we embrace this pluralism?
And notice again that it is not saying or pointing merely to the Holy Scriptures but to the very very thing they heard with their very ears, saw with their very eyes, experienced face to face.
2 Timothy 2:2
2And the things you have heard me say in the presence of many witnesses entrust to reliable men who will also be qualified to teach others.
How do you fulfill this scripture in the protestant expression if sola scriptura is your only authority?
The verse is not saying to entrust to men only what you read in the bible and believe what it means to say even though no two will read it the same and so entrust THAT to reliable men...
No. "the things you have heard me say" "entrust"
Philippians 4:9
9Whatever you have learned or received or heard from me, or seen in me—put it into practice. And the God of peace will be with you.
How do you do this outside of scripture if scripture is your only authority? Was what was learned, recieved, heard, seen and put into practice, lost?

Philemon 1:8
Paul's Plea for Onesimus
8Therefore, although in Christ I could be bold and order you to do what you ought to do
Philippians 2:2
2then make my joy complete by being like-minded, having the same love, being one in spirit and purpose.
How can you be like-minded or have the same love even if you can not be one in spirit or purpose?
Two contradictory teachings can't be right....
Romans 15:14
Paul the Minister to the Gentiles
14I myself am convinced, my brothers, that you yourselves are full of goodness, complete in knowledge and competent to instruct one another.
Complete in knowledge prior even to the arrival of the letter to the Romans. Hm. How can they have been complete in knowledge even prior to the epistle of Romans?
3 John 1:9
9I wrote to the church, but Diotrephes, who loves to be first, will have nothing to do with us.
If only Diotrephes had believe the Apostle to be authoritative and not saw himself as being authoritative (by what other authority would he have the audacity to reject such an authority but by himself?), perhaps we would have another epistle.
But nothing is lost. Because the fullness of the faith was deposited into the body even prior to the epistles having been written.
1 Corinthians 1:10
10I appeal to you, brothers, in the name of our Lord Jesus Christ, that all of you agree with one another so that there may be no divisions among you and that you may be perfectly united in mind and thought.
There are many many many more verses that point to this reality of which I could not reconcile in protestantism. I only urge the reader to seek first the Kingdom that all this might not remain such a burden to him. Seek the Truth. Love God with all your being and your neighbor as yourself and I am confident that the Lord will deliver you. I too hope to attain to the unity of the faith.
All things work out for good for those that love God.
John 14:15
"If you love me, you will obey what I command..."
God gives His authority to those who honor His Word.
Glory to Jesus Christ