Objection 1. It seems that created truth is eternal. For Augustine says (De Lib. Arbit. ii, 8) "Nothing is more eternal than the nature of a circle, and that two added to three make five." But the truth of these is a created truth. Therefore created truth is eternal.
Objection 2. Further, that which is always, is eternal. But universals are always and everywhere; therefore they are eternal. So therefore is truth, which is the most universal.
Objection 3. Further, it was always true that what is true in the present was to be in the future. But as the truth of a proposition regarding the present is a created truth, so is that of a proposition regarding the future. Therefore some created truth is eternal.
Objection 4. Further, all that is without beginning and end is eternal. But the truth of enunciables is without beginning and end; for if their truth had a beginning, since it was not before, it was true that truth was not, and true, of course, by reason of truth; so that truth was before it began to be. Similarly, if it be asserted that truth has an end, it follows that it is after it has ceased to be, for it will still be true that truth is not. Therefore truth is eternal.
On the contrary, God alone is eternal, as laid down before (10, 3).
I answer that, The truth of enunciations is no other than the truth of the intellect. For an enunciation resides in the intellect, and in speech. Now according as it is in the intellect it has truth of itself: but according as it is in speech, it is called enunciable truth, according as it signifies some truth of the intellect, not on account of any truth residing in the enunciation, as though in a subject. Thus urine is called healthy, not from any health within it but from the health of an animal which it indicates. In like manner it has been already said that things are called true from the truth of the intellect. Hence, if no intellect were eternal, no truth would be eternal. Now because only the divine intellect is eternal, in it alone truth has eternity. Nor does it follow from this that anything else but God is eternal; since the truth of the divine intellect is God Himself, as shown already (5).
Reply to Objection 1. The nature of a circle, and the fact that two and three make five, have eternity in the mind of God.
Reply to Objection 2. That something is always and everywhere, can be understood in two ways. In one way, as having in itself the power of extension to all time and to all places, as it belongs to God to be everywhere and always. In the other way as not having in itself determination to any place or time, as primary matter is said to be one, not because it has one form, but by the absence of all distinguishing form. In this manner all universals are said to be everywhere and always, in so far as universals are independent of place and time. It does not, however, follow from this that they are eternal, except in an intellect, if one exists that is eternal.
Reply to Objection 3. That which now is, was future, before it (actually) was; because it was in its cause that it would be. Hence, if the cause were removed, that thing's coming to be was not future. But the first cause is alone eternal. Hence it does not follow that it was always true that what now is would be, except in so far as its future being was in the sempiternal cause; and God alone is such a cause.
Reply to Objection 4. Because our intellect is not eternal, neither is the truth of enunciable propositions which are formed by us, eternal, but it had a beginning in time. Now before such truth existed, it was not true to say that such a truth did exist, except by reason of the divine intellect, wherein alone truth is eternal. But it is true now to say that that truth did not then exist: and this is true only by reason of the truth that is now in our intellect; and not by reason of any truth in the things. For this is truth concerning not-being; and not-being has not truth of itself, but only so far as our intellect apprehends it. Hence it is true to say that truth did not exist, in so far as we apprehend its not-being as preceding its being.
Source: Summa Theologica (New Advent)