The idea of lovers being "true" to each other also picks up on the image of lost faith from up above. Even if the world has lost its faith, maybe they, in their small way, can hold on to some of it.
But note the enjambment between lines 29 and 30. First, he says to his love, "let us be true." That could be a more general statement about personal integrity. But then, squish it up with line 30, and you realize that he wants them to be true to one another, which is a much smaller, more intimate idea.
Maybe, just maybe, the idea of being true in the modern world is just too big to handle. So in the end, all we can do is be true to one another. Let's see if that plays out in the final lines of the poem.