There was nothing wrong with offering grain to the Lord (Leviticus 2:14; Deuteronomy 26:2–4). But Cain only gave a small portion of his crops. God requires the first and best (Exodus 23:16, 19; 34:22, 26). Cain's attitude made his offering less valuable than Abel's (Hebrews 11:4). So, God did not accept Cain's offering.