We often here that God is eternal, but the word “eternal” can mean more than one thing.
On the one hand, it can mean everlasting–that is, something that endures through successive moments of time with either no beginning, no end, or both.
On the other hand, it can also mean atemporal–beyond or outside of time.
Catholic teaching holds that God is eternal in the second sense, but where could you go to show that?
You could quote from Aquinas on this point, but while Aquinas is very respected, including by the Magisterium, he is not himself an agent of the Magisterium.
But here is a passage which, because it is from one of John Paul II’s general audiences, is an exercise of the Church’s Magisterium:
These facts of revelation also express the rational conviction to which one comes when one considers that God is the subsisting Being, and therefore necessary, and therefore eternal.
Because he cannot not be, he cannot have beginning or end nor a succession of moments in the only and infinite act of his existence.
Right reason and revelation wonderfully converge on this point.
Being God, absolute fullness of being, (ipsum Esse subsistens), his eternity “inscribed in the terminology of being” must be understood as the “indivisible, perfect, and simultaneous possession of an unending life,” and therefore as the attribute of being absolutely “beyond time” [General Audience of Sept. 4, 1985].
So: “his eternity . . . must be understood as . . . being absolutely ‘beyond time.'”
Also note that the definition of eternity offered here the “indivisible, perfect, and simultaneous possession of an unending life,” is the classical definition proposed by the Christian philosopher Boethius in The Consolation of Philosophy, around the year A.D. 524:
It is the common judgement, then, of all creatures that live by reason that God is eternal. So let us consider the nature of eternity, for this will make clear to us both the nature of God and his manner of knowing. Eternity, then, is the complete, simultaneous and perfect possession of everlasting life; this will be clear from a comparison with creatures that exist in time.
…for it is one thing to progress like the world in Plato’s theory through everlasting life, and another thing to have embraced the whole of everlasting life in one simultaneous present. (Boethius Consolation, V.VI.) [MORE.]


Today is Wednesday of the 10th week of Ordinary Time. The liturgical color is white.
Today is Tuesday of the 10th week of Ordinary Time. The liturgical color is green.
Today is Monday of the 10th week of Ordinary Time. The liturgical color is red.
Today is the 10th Sunday of Ordinary Time. The liturgical color is white.