The research is to answer an old question about the reason for resending prophets and reinstating religion laws, from the viewpoint of traditional theologians and philosophers.
First, the necessity for sending prophets is examined, which includes the reasons like, Divine wisdom, improving human intellect, giving ultimatum, and offering a comprehensive law for most perfect human being. Then, the reasons for resending prophets and replacing religions are presented, which are consisted of: reviving natural spirit and the message of previous prophets, evolutionary development of human society, and falsification of Divine Scriptures.