This is only one of many of the theistic proofs. It goes like this:
1. An infinite regress of causes (a never ending series of causes or explanations) is illogical and therefore impossible.
2. This problem of infinite regress of causes can only be resolved if a first uncaused cause exists.
3. THEREFORE, an uncaused Causer must exist.
Premise #1: An infinite regress of causes is illogical and therefore impossible.
Any phenomenon requires a sufficient cause(s). However, if there is a causal infinite regress, there cannot be a sufficient cause(s). Cause (or explanation) X is never adequate to explain any phenomena because cause X requires the prior cause V, which requires its own cause V, ad infinitum (to infinity), implying that there is never an adequate explanation or any ultimate answer.
Also, since a causal series usually posits multiple causes for any one effect, the number if infinite series would be multiplied at each causal stage.
Besides, science has never identified a temporal (in this universe) uncaused cause, suggesting that there is no temporal cause that can resolve the problem of an infinite regress.
Also, actual infinities are logically impossible. An infinite number of causes requires an infinite amount of time or years. However, if this universe existed eternally, it would be impossible to ever arrive in the present. Why? Because accomplishing an infinite number of years would be required to ever arrive in present time from infinity in the past! However, it is impossible to count or fulfill an infinite number.
This suggests that both time and causation are finite, and that a first uncaused cause must exist. If science has concluded that the universe is only 14 billion years old, then temporal causation must also be limited by these years.
This means that an infinite causal series is not possible.
PREMISE #2: This problem of infinite regress of causes can only be resolved if a first uncaused cause exists.
This uncaused Cause can answer the problem of infinite regress. How? Because the buck stops there, with Him! He is the beginning of all causation and doesn’t Himself require a cause.
This Causer must be eternal. Otherwise, He too would require a cause. He must also exist beyond time, space, and matter. If this uncaused Causer is trapped within time, then He couldn’t be eternal. This is because time could not have existed eternally. Therefore, He must transcend time, space, and matter.
CONCLUSION: There does not seem to be any other way to resolve the problem of infinite regress, and this is a problem that requires resolution. Therefore, an uncaused Causer must exist.
This proof, of course, is not perfect. Our knowledge is limited, and there might exist another possible explanation. However, based upon our limited common-sense understanding, our Lord seems to be the only way to resolve the problem of an infinite regress.