Morgoth is NOT an allegory of Satan or any other evil deity which one could equate with him. He is Prof. Tolkien's personification of evil. Tolkien has denied and denied again that anything in the Lord of the Rings or the Silmarillion is representative of anything in the real world; it's not World War II, the Shire is not England, Gondor is not France, Morgoth is not Satan, the One Ring is not the Atom Bomb, and Long Bottom Leaf is NOT marijuana.

Tolkien created these worlds as an environment for his languages to live in, and greatly supported the idea that he was just a historian rewriting ancient tales of mythology (The Red Book of Westmarch).

In short, yes, Morgoth could be EQUIVALENT to a Satan, but should never be seen to represent Satan.

This node does a very good job at looking at the subject in depth.