Backtracking here a moment: They did not give equally nor did they both give in obedience. Theories and guesses are one thing, but to present the facts would better serve all reading this thread. And that only can be discovered by closely and more thoroughly combing through scripture to discover what is the answers to our questions; or at the least, present the best case scenario founded upon the evidence.


One: Abel did not bring grain, vegetable or fruit, but brought "of his flock"...meaning an animal; specifically meaning a sheep; a "firstling of his flock", ...meaning a young sheep, a lamb; ...which also means a 'clean' animal, as opposed to an unclean one (i.e. not swine). But he also brought "of the fat thereof" of that animal as part of the offering to God. That indicates that the offering was a sacrifice, a blood-sacrifice, ...even indicating a offering for sin - not a 'firstfruits' offering (i.e. the engathering of produce from the fileds) such as was commanded later under the Law (as was at Pentacost).


Two: It wan't that Cain's offering indicated a 'weaker faith'; but rather a lack of faith, a lack of honor and disrespect towards God - he did that which was not "well" or acceptable before God. He knew what to do, what was acceptable and unacceptable to 'bring' to God as an offering; he just chose to do what he wanted, and expected it to be accepted based upon the self-effort itself he put into it, it appears.

He was "wroth", "very wroth" and his "countenance fell" because he was evil and resented that God would not 'honor' and 'accept' (have respect for) what he brought. He was not ignorant of what was required - just proud, independent and self-absorbed. He did not want to honor God, he wanted God to honor him. 


Genesis 4 
1 And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD.
2 And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.
3 And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.
4 And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering:
5 But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.
6 And the LORD said unto Cain, Why art thou wroth? and why is thy countenance fallen?
7 If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.

Not really.  Didn't God tell Cain that it was because he didn't do well?

Ezekiel 36:22-32 says this: "Therefore say unto the house of Israel, Thus saith the Lord GOD; I do not this for your sakes, O house of Israel, but for mine holy name's sake, which ye have profaned among the heathen, whither ye went. And I will sanctify my great name, which was profaned among the heathen, which ye have profaned in the midst of them; and the heathen shall know that I am the LORD, saith the Lord GOD, when I shall be sanctified in you before their eyes. For I will take you from among the heathen, and gather you out of all countries, and will bring you into your own land. Then will I sprinkle clean water upon you, and ye shall be clean: from all your filthiness, and from all your idols, will I cleanse you. A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh. And I will put my spirit within you, and cause you to walk in my statutes, and ye shall keep my judgments, and do them.
And ye shall dwell in the land that I gave to your fathers; and ye shall be my people, and I will be your God. I will also save you from all your uncleannesses: and I will call for the corn, and will increase it, and lay no famine upon you.  And I will multiply the fruit of the tree, and the increase of the field, that ye shall receive no more reproach of famine among the heathen. Then shall ye remember your own evil ways, and your doings that were not good, and shall lothe yourselves in your own sight for your iniquities and for your abominations. Not for your sakes do I this , saith the Lord GOD, be it known unto you: be ashamed and confounded for your own ways, O house of Israel."

No where in these citations can there be found any intimation of man's contributions to the LORD's salvation plan. Paul didn't do "well" until the LORD saved him, and clearly he detested the whole idea and ideology of Christendom. Yet, the LORD made him His own, not the other way around. There is nothing Cain can do to please the LORD, in and of himself and under his own steam. The Holy Spirit has to be a part of this, or there is no salvation or regeneration and, consequently, there is nothing that Cain, Paul or anyone of us can do to please the LORD.

Genesis 4  
1 And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD.
2 And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.
3 And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.
4 And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering:
5 But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.
6 And the LORD said unto Cain, Why art thou wroth? and why is thy countenance fallen?
7 If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.

Seems clear to me that God placed it back squarely on Cain, not on some imagined unknown process for choosing those He chose.

In a word, obedience.

Blood