For thousands of years, the first cause argument has been the leading argument in favour of God. The argument is that you cannot have an infinite regression of events, and therefore in order for there to be a present, you must have a finite past. Therefore, there must be a first cause, an uncaused cause. The finger that flicked the first domino, if you will.
In Christianity, God is eternal. There was never a time when He did not exist, or when He was not God. For this reason, He is the perfect candidate for the first cause of the universe. But in Mormonism, there is an infinite regression of gods. Our God had a god. God's god had a god. God's god's god had a god. So on and so forth, extending back an infinite amount of time, at least according to Brigham Young (1). But this is mathematically impossible! You cannot have an infinite past, because otherwise you could not have a present. Think of it this way: If I asked you to press a button an infinite number of times, then I'd give you $1million, you'd never get the $1million. In the same way, if I told you to wait an infinite number of years, then you'd get the money, you still would not. So, if we were the end result of an infinite number of gods on an infinite number of worlds, spanning backwards through eternity, we could literally never exist. Therefore, the fact that we exist proves that there is a finite past.
Because both atheism and Mormonism necessitate an infinite amount of finite past events, Mormonism is just as impossible as atheism. By contrast, Christianity does not face this problem. With God, we do not need an infinite number of past events because time itself is an invention of God. God exists outside of time, and is thus immune to its effects. Therefore, Christian monotheism is mathematically superior to Mormon henotheism. Unless the Mormon wants to argue that you can reach the end of infinity (at which point my response would be "ok, do it, and I'll become a Mormon at the end), it's time to leave the Mormon Church and instead accept the true Gospel.
References
1. Young, Brigham - Discourses of Brigham Young, 1854