Why was Abel's sacrifice acceptable to God?
Abel's sacrifice was acceptable to God because Abel offered the required sacrifice as God had instructed, and he did so by faith. He believed and obeyed what God said to do, unlike Cain, who did not do what God said and thus lacked faith; without faith it is impossible to please God.
Les explains that Cain and Abel were likely of the same natural stock and probably shared similar values. Both had come to realize they had sinned, and God had told them what to do when sin came. Abel came with the required sacrifice as God had instructed it, because he had faith to do what God said to do. Cain came with something, but his big problem was that he did not do what God said; that made him without faith, and without faith it is impossible to please God. The same principle appears with the Ethiopian eunuch, who heard what God required for salvation and believed it with all his heart.