“Mystery” or “Sacred Secret”?
Rotherham’s Emphasized Bible calls the administration that we live in “…the administration of the sacred secret…,” while the NIV says “…the administration of this mystery….”Ephesians 3:9
And to bring to light—What is the administration of the sacred secret which had been hidden away from the ages in God, who did all things create.Ephesians 3:9 (NIV)
and to make plain to everyone the administration of this mystery, which for ages past was kept hidden in God, who created all things.
Why the difference? Throughout the history of the Christian Church, scholars and theologians have propounded doctrines they considered “mysteries,” and therefore the concept of the “mysterious” things of God has become part of Christian doctrine. Thus many versions of the Bible translate the Greek word musterion as “mystery.” This is unfortunate because musterion does not mean “mystery.” “Mystery” is a transliteration of the word musterion, not a translation of it. A “transliteration” is when the letters of a word in one language are brought across into another language. The prefix “trans” means “across,” and the Latin littera means “letter.” Thus, “transliteration” is literally “bringing across the letters.” In contrast, “translation” is bringing the meaning of a word in one language across into another language. If we are going to bring the meaning of the Greek into English, we must translate, not transliterate.
The English word “mystery” means something that is incomprehensible, beyond understanding, unknowable. Thus it is common in religious circles to speak of things such as the “Trinity” or Transubstantiation  as “mysteries” because they cannot be understood. In contrast, a “secret” is something that is known by someone but unknown by others.  A surprise birthday party is a “secret” to the person having the birthday, but known by those who will attend it. The Greek word musterion means “sacred secret,” that is, a secret in the sacred or spiritual realm that must be made known by God.
It is well documented by scholars that musterion refers to a secret, and not to our standard meaning of “mystery.”Musterion:
In the New Testament it denotes, not the mysterious (as with the English Word), but that which, being outside the range of unassisted natural apprehension, can be made known only by divine revelation, and is made known in a manner and at a time appointed by God. 
But whereas “mystery”
may mean, and in contemporary usage often does mean, a secret for which no answer can be found, this is not the connotation of the term mysterion in classical and biblical Greek. In the New Testament, mysterion signifies a secret which is being, or even has been, revealed, which is also divine in scope, and needs to be made known by God to men through his Spirit. 
The mystery of the New Testament has been described as an ‘open secret’; matters previously kept secret in God’s eternal purposes have now been or are being revealed (Eph. 3:3-5; 1 Cor. 2:7-8). 
Scholars’ assertion that the word musterion
does not mean “mystery,” but instead refers to a secret that can be known, is confirmed by many verses of Scripture. The verses quoted below make two points that the Bible student must know if he wants to understand the Administration of the Sacred Secret. The first is that the Sacred Secret was hidden from earlier generations and in earlier ages. The second is that God has now made it known. God began to reveal it after the resurrection of Christ and completed revealing it when He made it known to the Apostle Paul. Since Christians can know the Sacred Secret, it cannot be a “mystery” in the sense that most people think of today, i.e., something that cannot be known.Romans 16:25b and 26a
(25b) …the mystery [musterion] hidden for long ages past,
(26a) but now revealed and made known through the prophetic writings by the command of the eternal God…
more can be read on this here