Nobody. Christians hold that God has always existed and will always exist and is, therefore, uncaused. Only things that begin to exist need a cause, and God never began to exist, so God needs no cause. Nobody and no thing made God.
Something or someone had to have always existed, or else everything that exists now would have ultimately come from nothing. Nothing causes nothing, so the fact that something exists today means that something or someone must have always existed.
An infinite regress of causes going backward in time not only doesn’t solve the problem, it makes the problem infinitely worse. You are just adding an infinite number of effects that need a cause.
You have to stop somewhere with causation. Atheists may claim that the universe needs no cause, but if it began to exist, then it does need a cause. The atheist may respond that the universe never began to exist, and therefore does not need a cause. But this is a statement of faith.
Ultimately, you either go with God or matter, personality or impersonality, intelligence or non-intelligence, as the source of everything.