Has all the prophecy in the book come true?
Much prophecy has already come true, and everything that has not yet come true is certain to be fulfilled. The context says we can already see the handwriting on the wall, meaning it is about to happen, and affirms that if that much is true, it all is true.
Les explains that the book contains so much prophecy foretelling events hundreds and even thousands of years in advance, and it has all come true. For the prophecy that has not yet been fulfilled, he says we can already see the handwriting on the wall—it is about to happen. He reasons that if so much prophecy is true, then all of it must be true. Therefore, when we contemplate these things, we rest on the Word of God as absolute, true, and as sure as anything can be.