In proving that the theological virtue of hope is certain, Thomas formally says that hope is a virtue tending to beatitude, and all virtues achieve what they tend to with certitude. At the end of the proof, however, he adds a sort of addendum:
[H]ope tends to its end with certainty, as though sharing in the certainty of faith which is in the cognitive faculty.
This quotation suggests another argument, which might be fleshed out like this:
1.) Something is of the faith because it is revealed for the sake of salvation, that is, faith is God’s acting for salvation of those to whom he reveals.
2.) So if I now have faith then God now acts for my salvation, not in a general sense of acting for all men, but specifically by for acting to save me.
3.) Whatever God acts for, must come to pass.
4.) So if I now have faith, I must believe I would now be saved.
One can tune up some parts of the argument, but what is here seems to suffice to show why the certitude of my faith must be connected to the certitude of my hope of salvation.
Just Thomism 