Clement of Alexandria

And when it is said, "God is faithful," it is intimated that He is worthy to be believed when declaring aught. Now His Word declares; and "God" Himself is "faithful." How, then, if to believe is to suppose, do the philosophers think that what proceeds from themselves is sure? For the voluntary assent to a preceding demonstration is not supposition, but it is assent to something sure. Who is more powerful than God? Now unbelief is the feeble negative supposition of one opposed to Him: as incredulity is a condition which admits faith with difficulty.

That God is faithful means that we can trust his self-revelation. His Word reveals him. He is the God who is faithful.