Why can't the past be infinite? The answer is that it is impossible to complete an infinite series by addition. The series of past events is complete. Think of this mathematical fact. Why is it impossible to count to infinity? It is impossible because, no matter how long you count, you will always be at a finite number. It is impossible to complete an actual infinite by successive addition.
The past is complete. This claim means that the entire series of past events ends now. It ends today. Tomorrow is not part of the series of past events. The series of past events does not extend into the future. It is complete at the present. If it is impossible to complete an infinite series by successive addition (as it is impossible to count to infinity) the past cannot be infinite. If the past is finite., that is, if it had a beginning, then the universe had a beginning. We have strong philosophical reason to reject the claim that the universe has always existed.
I will not develop these. Rather, I will simply point them out.
Big Bang theory does not prove that the universe had a beginning, but it supports this claim.
The second law of thermodynamics does not prove that the universe had a beginning but it also supports this claim.
We can see that we have good philosophical and Scientific reasons to reject the idea that the Universe has always existed.
About the Universe, there are only three alternatives:
1. The universe has always existed. It has an infinite past.
The universe was popped into existence from nothing with absolutely no cause.
The universe was caused to exist by something outside it.
We have strong reason to reject the first two alternatives.
Alternative Three is the most reasonable. There was a first cause. This cause existed eternally. It initiated the big bang and created the universe. Now what can we know about this cause? Why think the cause is God? I will briefly sketch a few implications.
First, the first cause is not a part of the space-time physical universe because it caused the space time universe to begin. Therefore it is outside of space and time. It is not physical. Second, it has a great deal of power. Third, it is a personal agent. This means it is not an inert force but it must have aspects of person hood; namely, that it wills. How do we know this? This is because it is the best answer to the question of why the Big Bang happened when it did. Why not sooner? Why not later? All of the conditions for producing the Big Bang existed from eternity. The only kind of cause we know of that can initiate an effect when all of the conditions are already present is the will of a personal agent.
I have not argued that it is logically impossible that the universe popped into existence from nothing without cause. I have argued that it is more reasonable to hold that it has a cause and that this cause is a non-physical personal agent -- God.
So it seems that the first argument is fairly strong. The existence of the universe is better explained by the existence of God.