Q: Gen 4: 3-5 God looked favorable upon Abel’s offering but not Cain’s. Why? It doesn’t seem fair to me.
A: Actually verse 7 gives us a clue. Cain was really not trying to do what was right but just trying to get by and do his own thing. Verse 3 says that he merely brought some fruit as an offering while it says in verse 4 that Abel selected his offering the best fat portions from the first born of his flock. So on one hand there is an attitude difference.
Secondly, a blood sacrifice was what God instituted as an offering for sin. When Adam and Eve fell in the Garden God gave them the animal skins to wear. That was the first blood sacrifice for sin.
So in essence Cain was really starting his own religion here—the First Church of Fruit Sacrifice.
The blood sacrifice was an important feature of ancient religion instituted by God which was suppose to be a foreshadowing of the ultimate sacrifice of God’s own Son on the cross.