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Collect: O God whose existence is essential, grant that we may come to contemplate your necessarily existence and thus continuously be unmovably certain by faith, through Christ our Lord. Amen.
In the Sed Contra, St. Thomas gives an interesting proof of the thesis at hand. Many may view St. Thomas' response as unconvincing and "weak," but once we grasp the true nature of the argument it makes more sense. For, the argument is not meant to be "apodictic," i.e., infallibly and necessarily follow, rather, it is meant to be a merely probable argument. For, we can think of many reasons why a self-evident principle would be denied by many, e.g., they do not understand the terms. Rather, St. Thomas is arguing from what would be most likely, i.e., "if the existence of God was self-evident, we would expect nobody to mentally deny the proposition, but many do."
It is also further to note something about the state of the question that will come up in the respondeo. St. Thomas is trying to prove that the existence of God is not self-evident quoad nos (i.e., to us). Thus, the argument of St. Thomas, on these terms, gains a great deal of probative force, for, those who deny the existence of God is included under "us."