What is the ruling on the multiplicity of religions since the dawn of creation, and why have prophets not multiplied generation after generation to spread the religion of Islam, and why did Christianity, Sabianism, and others come into being?

The religion of Allah is one, and that is Islam. All prophets, peace be upon them, brought it, and it was consistent in its fundamentals, even if some laws differed. The message of our Prophet Muhammad, peace and blessings be upon him, is the final message and abrogates what came before it. Allah does not accept from anyone who has heard of it except that they believe in him and follow him. Thus, Christianity or Sabianism are no longer valid religions.