Fundamentals of Catholic Dogma, Ludwig Ott, pp. 36-37.
§ 18. God’s Eternity
Eternity is a duration without beginning and without end, without sooner and later, a “permanent now” (nunc stans). The essence of eternity is the absolute lack of succession. Boethius gave the classical definition: Aeternitas est interminabilis vitae tota simul et perfecta possessio (Eternity is the perfect and simultaneous total possession of interminable life) (De consol. phil. V. 6). From eternity in the strict sense must be distinguished the “aevum” or the “aeviternitas,” that is, the duration of the created spirits, which have indeed a beginning, but no end, and which, in their substance, are subject to no mutation.
God is eternal. (De fide.)
“The dogma asserts that God possesses the Divine Being without beginning and without end, and without succession in a constant undivided now. The Symbolorum Quincumque declares: Aeternus Pater, Aeternus Filius, Aetemus Spiritus Sanetus et tamen non tres aeteruni, sed unus aeternus. (Eternal Father, Eternal Son, Eternal Holy Ghost and yet not three eternal beings but one.) D 39. The 4th Lateran Council and the Vatican Council attribute to God the predicate” eternal" (aeternus). D 428, 1782.
Holy Writ bears witness to the individual grounds of the Divine eternity. The negation of beginning and end is expressed in Ps. 89, 2: II Before the mountains were made, or the earth and the world were formed: from eternity and to eternity Thou art God." The absolute lack of succession is seen in Ps. 2, 7: “The Lord hath said to me: Thou art my Son, this day have I begotten thee.” Jo. 8, 58: “Before Abrahatnm was made, I am.” Cf. Ps. 101, 27 et seq.; 89, 4; 2 Peter 3, 8.
The Fathers, in their conflict with the heathen world, familiar ,with the genealogies of gods, expressly attest God’s eternity. Cf. Aristiotle’s, Apol. 1.4; Tatian, Or. 4,3 ; Athenagoras Suppl. 10; St. Irenaeus, Adv. haer. II 31, 2. St. Augustine says that God’s eternity is a constant present: “The eternity of God is His Essence itself: which has nothing mutable in it. In It there is nothing past, as if it were no longer, nothing future, as if it had not yet been. In It there is only , is,’ that is, the present” (Enarr. in Ps. 101, 2, 10).