Paul says that Abel, through faith, offered a more excellent offering than Cain (Hebrews 11:4), but we aren’t told what Paul means by a more excellent offering. Notice what was done:
And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD. 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: But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell. (Genesis 4:3-5 KJV)
The text tells us that in some manner God showed his respect for Abel and his offering, but not so for Cain and his offering (Genesis 4:3-5). In other words, whatever God did, perhaps sending fire down from heaven to consume Abel’s sacrifice (viz. 1Kings 18:36-39), testified on Abel’s behalf that he was righteous. Many assume that the more excellent offering was the fact that Abel showed his obedience to God through a blood sacrifice for a sin offering (Exodus 29:14, 36; 30:10), pointing to Christ (cf. Genesis 3:15), while what Cain offered was nothing more than a fellowship offering (Exodus 18:12; Leviticus 2:1-3). However, if this is the case, it would be necessary for us to read that into the text, because, the Law, which describes all the offerings and their place in man’s relationship with God, wasn’t given until Moses. Nowhere in the Scriptures of Genesis do we find God detailing what sort of sacrifice would be acceptable. What we are told in the text is that Abel’s sacrifice was out of the firstlings of his flock, i.e. out of the best he had to offer. This is not said about Cain’s offering. We are left to assume he merely brought a nondescript offering out of what he had available. Both brothers offered a sacrifice according to his own chosen vocation.
If faith is the confidence of things hoped for, the conviction of things not seen, and, if faith, not the substance that Cain and Abel offered as a sacrifice, is the reason for the Lord’s respect, then it is clear, according to Hebrews 11:4, that Abel had confidence in the Lord to fulfill his hopes, and was convicted of things not seen (i.e. what the Lord had promised, viz. Genesis 3:15). Nevertheless, Cain, although he, evidently, feared the Lord, he had no confidence in him to fulfill his hopes or to keep the promises he (i.e. the Lord) had made. This, not the substance that was offered, is the reason for the Lord’s lack of respect for Cain’s offering.
Since we are informed in the text that the Lord testified of Abel’s gifts (plural), we can assume that Abel continued to offer sacrifices of the firstlings of his flock, and the Lord continued to show respect for him and his offerings (gifts). It is in this context that what God did became a point of contention between Cain and his brother, Abel. In fact, the text goes on to say in Genesis 4:8 that Cain told his brother about it. In other words, Cain began to criticize his brother’s righteous behavior. Nevertheless, Abel continued to express his faith in the Lord (i.e. faith = **it** in the text), and his righteous behavior was the reason behind his persecution and eventual death at the hand of his brother.
The fact that Cain seems to have been criticizing his brother’s walk with the Lord and eventually killing him over it, is meant by Paul to encourage first century AD believing Jews, who were, at the time of Paul’s writing, being intimidated by unbelieving Jews for their own righteous behavior. Thus, just as the Lord had respect for Abel and his offering, Paul implied that the Lord would also show respect for the believing Jews who were faithful to the Lord in spite of the persecution they were forced to endure.
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