[yowzadave]

In the case that Lewis is alluding to (an unsatisfied yearning for spiritual fulfillment), the argument is tautological. If God is real in the way he imagines it, the presence of this spiritual yearning is a kind of evidence. If God is not real in the way he imagines, the presence of this spiritual yearning is evidence that the assertion is false.

[feoren]

Also: spiritual yearning can in fact be satisfied by spiritual practices. So the conclusion doesn't follow even if you accept his premise.