Aquinas on the A- and B-Series

Summa Theologica I:14:15,

Whether the knowledge of God is variable?

Objection 3. Further, God knew that Christ would be born. But He does not know now that Christ will be born; because Christ is not to be born in the future. Therefore God does not know everything He once knew; and thus the knowledge of God is variable.…

Reply to Objection 3. The ancient Nominalists said that it was the same thing to say "Christ is born" and "will be born" and "was born"; because the same thing is signified by these three--viz. the nativity of Christ. Therefore it follows, they said, that whatever God knew, He knows; because now He knows that Christ is born, which means the same thing as that Christ will be born. This opinion, however, is false; both because the diversity in the parts of a sentence causes a diversity of enunciations; and because it would follow that a proposition which is true once would be always true; which is contrary to what the Philosopher lays down (Categor. iii) when he says that this sentence, "Socrates sits," is true when he is sitting, and false when he rises up. Therefore, it must be conceded that this proposition is not true, "Whatever God knew He knows," if referred to enunciable propositions. But because of this, it does not follow that the knowledge of God is variable. For as it is without variation in the divine knowledge that God knows one and the same thing sometime to be, and sometime not to be, so it is without variation in the divine knowledge that God knows an enunciable proposition is sometime true, and sometime false. The knowledge of God, however, would be variable if He knew enunciable things by way of enunciation, by composition and division, as occurs in our intellect. Hence our knowledge varies either as regards truth and falsity, for example, if when either as regards truth and falsity, for example, if when a thing suffers change we retained the same opinion about it; or as regards diverse opinions, as if we first thought that anyone was sitting, and afterwards thought that he was not sitting; neither of which can be in God.