5 While my mind was dwelling on these and on many like thoughts, I chanced upon the books which, according to the tradition of the Hebrew faith, were written by Moses and the prophets, and found in these words spoken by God the Creator testifying of Himself 'I Am that I Am, and again, He that is has sent me unto you.' I confess that I was amazed to find in them an indication concerning God so exact that it expressed in the terms best adapted to human understanding an unattainable insight into the mystery of the Divine nature. For no property of God which the mind can grasp is more characteristic of Him than existence, since existence, in the absolute sense, cannot be predicated of that which shall come to an end, or of that which has had a beginning, and He who now joins continuity of being with the possession of perfect felicity could not in the past, nor can in the future, be non-existent; for whatsoever is Divine can neither be originated nor destroyed. Wherefore, since God's eternity is inseparable from Himself, it was worthy of Him to reveal this one thing, that He is, as the assurance of His absolute eternity.
Source: On the Trinity (New Advent)