Ch3 of the W.C.of F. says "neither is God the author of sin".

One can make something which is entirely different from oneself. e.g. if someone makes a cake they do not become a cake themselves.
If God had a holy motive why Jesus alone should fulfil the Law {total obedience} then God might have a holy motive to withhold from man the grace to obey His Law/command. I can find no place where the writers of the W.C.of F. tried to justify the statement "neither is God the author of sin". Did they ever try to justify it?