You can’t act as if you can’t act, for example. So, it is not true that you can’t act. Likewise, you can’t think that you can’t think; can’t be aware that you can’t be aware; can’t mean that there is no meaning; can’t yourself suffer the illusion that your self is an illusion; and so forth.
This is the practical aspect of the fundamental epistemological criterion of truth, which is adequacy to quotidian experience.
Extending this notion a bit further: you can’t say that there is no such thing as metaphysical truth other than by asserting a putative metaphysical truth. Ditto for moral truths, and aesthetic truths: you can’t say that morals or aesthetics are relative except by asserting a moral or aesthetic absolute. Indeed, this holds for any sort of truth. You can’t say there is no political truth other than by asserting a putative political truth, for example.
Nominalism and positivism both fall before this scythe. Nominalism can be asserted only by means of the very universals it reprehends. Positivism itself is among the propositional systems that cannot be logically or empirically demonstrated, and insists are therefore meaningless; so that its assertion is its contradiction.
Also, of course, you can’t for very long successfully live as if an important falsehood were true. We’ve all proved this for ourselves a million times.
Thus the very rejection of God is an implicit recognition of him. You can’t rebel against a nonexistent Lord.