Who/What was Tom Bombadil

I don't agree with the theory that Tom was a Maiar like Gandalf. If that were true,he would not have been able to handle the one ring without it having an evil effect on him...the reason why Gandalf will not touch it. He is also not an Elf,as the ring probably would have sunk its "claws" into him and he would not want to return it to Frodo.

Tom although an enigma...must be some sort of supernatural being who is more powerful than Sauron,and the Maiar since the ring has no effect on him. He also laughs at it as if its pathetic!

I originally though he may be an incarnation Eru(in disguise so to speak). But if Tolkien said in his letters Eru never walked on Middle Earth,than that can't be so. I guess he'll always be an enigma...although we can at least deduce that he is very powerful,and probably supernatural in origin like the Maiar,although more powerful or at least less corruptible.