Faith produces action but not all action is faith in God. A wolf-in-sheep clothing can fake action but his faith is not in God.

Cain and Abel both sacrificed to God, but God accepted Abel's but rejected Cain's, because God can see the heart and see Abel's faith (in God), but saw Cain's heart was wicked. God could see past the actions and see the heart and determine if the actions were truly out of faith (in God) or mere acting.