Det finns två faktorer:
Den viktigaste delen är tidigare i brevet:
[Sauron] had gone the way of all tyrants: beginning well, at least on the level that while desiring to order all things according to his own wisdom he still at first considered the (economic) well-being of other inhabitants of the Earth. But he went further than human tyrants in pride and the lust for domination, being in origin an immortal (angelic) spirit. [...] Sauron desired to be a God-King, and was held to be this by his servants; if he had been victorious he would have demanded divine honour from all rational creatures and absolute temporal power over the whole world.
The Letters of J.R.R. Tolkien 183: Notes on W.H. Auden's review of The Return of the King. 1956
Sauron trodde nästan inte att han var verkligen Morgoth; i alla fall säger Tolkien i andra skrifter att han var en professor ateist, men inte en sann (icke) troende:
Sauron was not a 'sincere' atheist, but he preached atheism, because it weakened resistance to himself (and he had ceased to fear God's action in Arda). [...] To wean one of the God-fearing from their allegiance it is best to propound another unseen object of allegiance and another hope of benefits; propound to him a Lord who will sanction what he desires and not forbid it. Sauron, apparently a defeated rival for world-power, now a mere hostage, can hardly propound himself; but as the former servant and disciple of Melkor, the worship of Melkor will raise him from hostage to high priest.
History of Middle-earth X Morgoth's Ring Part 5: "Myths Transformed" Chapter VII: "Notes on motives in the Silmarillion" (i)
Tolkien skriver detta från Saurons tid som fånge i Númenor, där han ville korrupta den fruktansvärda befolkningen; Att presentera en alternativ gud är det bästa sättet att göra det, och Melkor är det bästa valet - Sauron själv är knappast i maktposition.
Efter sin "flykt" är det dock en annan historia; Sauron är plötsligt i en maktposition (mer i tredje ålder då han antogs förstördes och sedan återvände - det är en seriös gudkredit), så varför skulle inte