Sauron had not served
Morgoth, even in his last stages, without becoming infected by his lust for destruction, and his hatred of
God (which must end in nihilism). Sauron could not, of course, be a 'sincere' atheist. Though one of the minor
spirits created before the world, he knew
Eru, according to his measure. He probably deluded himself with the notion that the Valar (including
Melkor) having failed, Eru had simply abandoned Eä, or at any rate
Arda, and would not concern himself with it any more. It would appear that he interpreted the 'change of the world' at the Downfall of
Númenor, when
Aman was removed from the physical world, in this sense: Valar (and
Elves) were removed from effective control, and
Men under God's curse and wrath. If he thought about the Istari, especially Saruman and Gandalf, he imagined them as emissaries from the Valar, seeking to establish their lost power again and 'colonize' Middle-earth, as a mere effort of defeated imperialists (without knowledge or sanction of Eru). His cynicism, which (sincerely) regarded the motives of
Manwë as precisely the same as his own, seemed fully justified in Saruman. Gandalf he did not understand. But certainly he had already become
evil, and therefore stupid, enough to imagine that his different behaviour was due simply to weaker intelligence and lack of firm masterful purpose. He was only a rather cleverer
Radagast - cleverer, because it is more profitable (more productive of power) to become absorbed in the study of people than of
animals.