Here's a quote from the best book on the thought of St. Thomas Aquinas, which is, in my opinion, Etienne Gilson's The Christian Philosophy of St. Thomas Aquinas. This broke my brain.

   When St. Thomas substituted the point of view of existence for that of essence he found that he had not only to look for new proof for the existence of God, but that he had, above all, to emphasize the fact that the existence of God requires demonstration properly so-called. It is therefore the specificity of the existence of God as a problem which is immediately affirmed in his doctrine, as against the reduction of the problem to that of the divine essentiality in the theologies of essence. Most significant in this regard is the attitude St. Thomas adopts in his Commentary on the Sentences. No more than the others does he here undertake to demonstrate the existence of God because this task is not forced upon Commentators of Peter Lombard. But, at the very place where the Summa of Alexander [of Hales] and the Commentary of St. Bonaventure undertake to show that the existence of God is evident, St. Thomas devotes an article to proving that it is not.

Wow. You just never really know what the heck is going on in philosophy until you sit at the feet of real masters, like Gilson, who lays out the drama for you. Aquinas rejected the Platonic-Augustinian-Anselmian-Lombardian-Halesian-Bonaventuran-Avorresan consensus that God's existence is an accidental property of His divine essence. Basically, it was St. Thomas versus human history, and Tommy won.

They, the essentialists, thought that once an individual understood the essence of the divine nature, then it was evident that God exists. "God is that Being in which none greater can be conceived" said Anselm, and thus, God must exist, because to exist is greater than not existing, and since God is, by definition, that Being in which none greater can be conceived, He must therefore exist, by virtue of His essence.

But once Aquinas did away with an essentialist methodology, his existentialism demanded demonstration, real proof. Hence the Five Ways of St. Thomas Aquinas. They start with actually existing beings and proceed back to their origin by proofs, by demonstration, and not merely dialectic.