When St. Thomas Aquinas gave his proofs for God’s existence, his conclusion, which represents Catholic doctrine on the matter, is that God is the only being who must exist. The only thing for which existence is part of its very nature.
God is also eternal. We commonly have a misconception of eternity as being “an endless amount of time.” As St. Thomas explained, however, eternity is actually the absence of time. Therefore the terms, “beginning” and “end,” simply do not apply to Him. This is a great mystery to us - meaning that it is something we cannot possibly fully understand (at least, not in this life) - because we, and the universe in which we live exist in time. Likewise, “change,” which indicates the “end” of one state and the “begininng” of another, cannot apply to God.
God is our beginning and end, but has not beginning or end.
As God described himself: I AM. I AM WHO AM.
There are several other concepts St. Thomas explained regarding God’s existence that show the consistency and reasonableness of this belief. I would recommend “A Companion to the Summa,” by Father Farrell (out of print, but you can still find copies on
Amazon.com) for a “contemporary” (1950’s) explanation of St. Thomas’ teachings on this.
