Objection 1. It seems that God is not truth. For truth consists in the intellect composing and dividing. But in God there is not composition and division. Therefore in Him there is not truth.
Objection 2. Further, truth, according to Augustine (De Vera Relig. xxxvi) is a "likeness to the principle." But in God there is no likeness to a principle. Therefore in God there is not truth.
Objection 3. Further, whatever is said of God, is said of Him as of the first cause of all things; thus the being of God is the cause of all being; and His goodness the cause of all good. If therefore there is truth in God, all truth will be from Him. But it is true that someone sins. Therefore this will be from God; which is evidently false.
On the contrary, Our Lord says, "I am the Way, the Truth, and the Life" (John 14:6).
I answer that, As said above (Article 1), truth is found in the intellect according as it apprehends a thing as it is; and in things according as they have being conformable to an intellect. This is to the greatest degree found in God. For His being is not only conformed to His intellect, but it is the very act of His intellect; and His act of understanding is the measure and cause of every other being and of every other intellect, and He Himself is His own existence and act of understanding. Whence it follows not only that truth is in Him, but that He is truth itself, and the sovereign and first truth.
Reply to Objection 1. Although in the divine intellect there is neither composition nor division, yet in His simple act of intelligence He judges of all things and knows all things complex; and thus there is truth in His intellect.
Reply to Objection 2. The truth of our intellect is according to its conformity with its principle, that is to say, to the things from which it receives knowledge. The truth also of things is according to their conformity with their principle, namely, the divine intellect. Now this cannot be said, properly speaking, of divine truth; unless perhaps in so far as truth is appropriated to the Son, Who has a principle. But if we speak of divine truth in its essence, we cannot understand this unless the affirmative must be resolved into the negative, as when one says: "the Father is of Himself, because He is not from another." Similarly, the divine truth can be called a "likeness to the principle," inasmuch as His existence is not dissimilar to His intellect.
Reply to Objection 3. Not-being and privation have no truth of themselves, but only in the apprehension of the intellect. Now all apprehension of the intellect is from God. Hence all the truth that exists in the statement--"that a person commits fornication is true"--is entirely from God. But to argue, "Therefore that this person fornicates is from God", is a fallacy of Accident.
Source: Summa Theologica (New Advent)