Sauron's loyalty after the First Age

[Belegorn]

I think some people put forward the argument of Sauron not being loyal since instead of freeing Morgoth he took up the mantle of Dark Lord himself. "Morgoth and his satellite Sauron" [Letter 156] were like mirror images. Do you really think Sauron, even if he worked with Durin's Bane, had the ability to free Morgoth from the punishment of the Valar? I think you ask an impossible and foolhardy task to prove his loyalty. Morgoth had, "secret friends and spies among the Maiar whom he had converted to his cause, and of these the chief, as after became known, was Sauron, a great craftsman of the household of Aule." [MR, p. 52] He followed Morgoth, "because of his admiration of strength" and soon in the 2nd Age, "he assumed the position of Morgoth's representative" and then, "claimed to be Morgoth returned" at the end of the 3rd Age. Sauron, "represents as near an approach to the wholly evil will as possible" [Letter 183]. In another letter we are told how Sauron is attached to Morgoth as Gandalf is to Manwe. Also as Gandalf was his opposite so was Manwe, Morgoth's.