Not tryna start a theological debate or anything, but I think the argument is if the Universe has been around forever, then it didn't have a cause and there would be infinite pasts. You could never reach this point in time because there's an infinity behind you. This problem of infinite divisibility occurs in space, as well. How can you reach a point in space if you can keep dividing the distance infinitely? The solution is the plank length. Likewise, we know the universe traverses time and we are in the present, therefore it must of had a cause and beginning; which we know from the big bang theory that time itself had a beginning. God however can exist without a cause because he resides separately from the universe that traverses time and can be a cause because he has a will.