27 For we can embrace all time in imagination or knowledge, since we know that what is now today, did not exist yesterday, because what was yesterday is not now; and on the other hand what is now, is only now and was not also yesterday. And by imagination we can so span the past that we have no doubt that before some city was founded, there existed a time in which that city had not been founded. Since, therefore, all time is the sphere of knowledge or imagination, we judge of it by the perceptions of human reason; hence we are considered to have reasonably asserted about anything, “It was not, before it was born,” since antecedent time is prior to the origin of every single thing.
But on the other hand, since in things of God, that is to say, in regard to the birth of God, there is nothing that is not before time eternal: it is illogical to use of Him the phrase “before He was born,” or to suppose that He Who possesses before times eternal the eternal promise, is merely (in the language of the blessed Apostle) in hope of eternal life, which God Who cannot lie has promised before times eternal, or to say that once He was not. For reason rejects the notion that He began to exist after anything, Who, so we must confess, existed before times eternal.
Source: On the Trinity (New Advent)