Objection 1. It would seem that an exclusive diction can be joined to the personal term, even though the predicate is common. For our Lord speaking to the Father, said: "That they may know Thee, the only true God" (John 17:3). Therefore the Father alone is true God.
Objection 2. Further, He said: "No one knows the Son but the Father" (Matthew 11:27); which means that the Father alone knows the Son. But to know the Son is common (to the persons). Therefore the same conclusion follows.
Objection 3. Further, an exclusive diction does not exclude what enters into the concept of the term to which it is joined. Hence it does not exclude the part, nor the universal; for it does not follow that if we say "Socrates alone is white," that therefore "his hand is not white," or that "man is not white." But one person is in the concept of another; as the Father is in the concept of the Son; and conversely. Therefore, when we say, The Father alone is God, we do not exclude the Son, nor the Holy Ghost; so that such a mode of speaking is true.
Objection 4. Further, the Church sings: "Thou alone art Most High, O Jesus Christ."
On the contrary, This proposition "The Father alone is God" includes two assertions--namely, that the Father is God, and that no other besides the Father is God. But this second proposition is false, for the Son is another from the Father, and He is God. Therefore this is false, The Father alone is God; and the same of the like sayings.
I answer that, When we say, "The Father alone is God," such a proposition can be taken in several senses. If "alone" means solitude in the Father, it is false in a categorematical sense; but if taken in a syncategorematical sense it can again be understood in several ways. For if it exclude (all others) from the form of the subject, it is true, the sense being "the Father alone is God"--that is, "He who with no other is the Father, is God." In this way Augustine expounds when he says (De Trin. vi, 6): "We say the Father alone, not because He is separate from the Son, or from the Holy Ghost, but because they are not the Father together with Him." This, however, is not the usual way of speaking, unless we understand another implication, as though we said "He who alone is called the Father is God." But in the strict sense the exclusion affects the predicate. And thus the proposition is false if it excludes another in the masculine sense; but true if it excludes it in the neuter sense; because the Son is another person than the Father, but not another thing; and the same applies to the Holy Ghost. But because this diction "alone," properly speaking, refers to the subject, it tends to exclude another Person rather than other things. Hence such a way of speaking is not to be taken too literally, but it should be piously expounded, whenever we find it in an authentic work.
Reply to Objection 1. When we say, "Thee the only true God," we do not understand it as referring to the person of the Father, but to the whole Trinity, as Augustine expounds (De Trin. vi, 9). Or, if understood of the person of the Father, the other persons are not excluded by reason of the unity of essence; in so far as the word "only" excludes another thing, as above explained.
The same Reply can be given to Objection 2. For an essential term applied to the Father does not exclude the Son or the Holy Ghost, by reason of the unity of essence. Hence we must understand that in the text quoted the term "no one" [Nemo = non-homo, i.e. no man] is not the same as "no man," which the word itself would seem to signify (for the person of the Father could not be excepted), but is taken according to the usual way of speaking in a distributive sense, to mean any rational nature.
Reply to Objection 3. The exclusive diction does not exclude what enters into the concept of the term to which it is adjoined, if they do not differ in "suppositum," as part and universal. But the Son differs in "suppositum" from the Father; and so there is no parity.
Reply to Objection 4. We do not say absolutely that the Son alone is Most High; but that He alone is Most High "with the Holy Ghost, in the glory of God the Father."
Source: Summa Theologica (New Advent)