Genesis 4:1-2 Adam made love to his wife Eve, and she became pregnant and gave birth to Cain. . . . Later she gave birth to his brother Abel.
Cain and Abel were the first two sons of Adam and Eve. There were not many career choices back then, so Cain grew up to be a farmer, and Abel kept flocks. Cain brought some of his produce as an offering to God, and Abel brought fat portions from some of the firstborn of his flock (Genesis 4:4). This is how they honored God as the source of all they had.
Genesis 4:4-5 The Lord looked with favor on Abel and his offering, but on Cain and his offering he did not look with favor.
From this it appears that not all offerings are pleasing to God. The text is not explicit, but Abel seems to have brought his best, while Cain brought something less. The real problem was not what they brought, however, but the attitude of their hearts. Elsewhere we learn that Abel’s heart was right with God (Hebrews 11:4), and Cain’s heart was not (1 John 3:12). It is not enough to bring our offering to God; we must bring it with a heart of worship.