“We find in nature things that are possible to be and not to be, since they are found to be generated, and to be corrupted, and consequently, it is impossible for them to be and not to be. But it is impossible for these always to exist, for that which can not-be at some time is not. Therefore, if anything can not-be, then at one time there was nothing in existence.
If anything can not-be, then at one time there was nothing in existence
Now if this were true, even now there would be nothing in existence, because that which does not exist begins to exist only through something already existing. Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence – which is absurd.
Therefore, not all beings are merely possible, but there must exist something in the existence of which is necessary.
But every necessary thing either has its necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another, as has been already proved in regard to efficient causes. Therefore we cannot but admit the existence of some being having of its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God.”
– St Thomas Aquinas, Summa Theologica, in The Path from Science to God, Faith Pamphlets