Complex sentence: Each made sacrifices to God, but God only accepted Abel's sacrifice and offering, and not Cain's because Abel gave the "best" of his flocks, indicating that God came first in his heart.
Split into simpler sentences: Each made sacrifices to God, but God only accepted Abel's sacrifice, and not Cain's. God accepts Abel's offering and not Cain's because Abel gave the "best" of his flocks, indicating that God came first in his heart.