Why was the man rejected when he brought the sacrifice just because his neighbor had done it?
The man was rejected because he offered the sacrifice merely because his neighbor had done it, not by faith and not because God had instructed him to do so. Going through the motions without faith did not make him accepted.
Les explains that acceptance was not based on the outward act alone. If the man only admitted his sin but refused to bring what God required to the priest, he was not accepted, because he did not do what God said. If he then brought a sacrifice merely because his neighbor had done it, he was still not accepted, because he did not do it by faith or because God instructed it. Going through the motions without faith was not accepted, and if he had faith but did not carry out the work, he was also not accepted. That was legalism; that was the law.