3. Therefore, there exists a Supreme Truth. (From 1 and 2)
In support of (1), I'm reminded of C.S. Lewis's famous analogy: one cannot know a line is crooked without having some idea of what a straight line looks like.
Premise (2) should be obvious to anyone who doesn't claim omniscience. Based on these two premises, (3) necessarily follows. However, what's the significance of stating there exists a Supreme Truth? As far as I can tell, propositional truth is an abstract object, but no classical theist equates God with a mere abstraction.
One solution to this problem is to postulate that abstract objects, such as truth, exist as mental concepts, as opposed to mind-independent realities (the latter of which Plato held, whereas St. Augustine postulated the former). If conceptualism is true, then the Supreme Truth, which includes necessary and contingent truths, would have to be the concept of a necessary and omniscient mind, e.g. God.