A chief theological difference between mainstream Christianity and Mormonism is found in “Creation Ex Nihilo”. This is the belief that God is a non-created being who created the universe out of nothing (or out of himself). In my understanding, Mormonism holds that God may or may not be created and that he organized the universe with pre-existing materials.
The most obvious criticism of the Mormon viewpoint is that it leads to the logical fallacy of the endless regress. If God is created, who created God? And who created that creator? And who created that creator? etc. The answer to each question is just passing the buck on into the past.
We know that in the laws of nature (something Evangelicals would say was introduced in the creation) that something can not come from nothing. So Mormons need to answer “where did the pre-existing natural materials come from that God used to create?” And I would follow that question with “if they were created, why aren’t we worshiping their creator?”
The mainstream Christian view is probably most strongly supported by John 1. Aristotle and Plato’s asked the question: who is the logical necessity that is the uncaused cause or the unmoved mover? John answers “in the beginning was the Word, and the Word was with God, and the Word was God. He was with God in the beginning.Through him all things were made; without him nothing was made that has been made.” Later John makes it clear that the Word is Jesus. (Which raises more problems for Mormon theology about the nature of Jesus.)
The idea of creation ex-nihilo is supported by the evidence of the Big Bang. The Big Bang shows that the universe had a beginning. There was a one time a specific point where all things were born and put into motion. If it had a beginning that means in had to have a “beginner”.
I’m seriously short changing these concepts and ideas. (and there’s likely going to be some fledgling BYU philosophy student who wants to comment with their own 30 page retort). If you like to read more I’d recommend William Lane Craig’s excellent article in “The New Mormon Challenge” where he specifically places his cosmological argument in the context of Mormon studies.