<Snipped quote by Spinosaurus>
Well... we aren't sure as of now. Truthfully, we don't know that much about Doomguy's personal life before the events of the games. The litch person you speak of, it's actually known as the "the wretch", probably a traitor to Hell and the Demons.
However, while the demons' scriptures claim that the armor was made in hell, closer examination reveals English text and even logos for human manufactures, suggesting the armor was actually made on Earth, but somehow came into hell and was perhaps given to the Doom Marine then.
Again, we simply don't truly know at this stage. Perhaps if a sequel is made, we can get more info. This is all that we really know of the Praetor suit as of now.
Edit: It could also be the same Doomguy character we see in the original games, since according to texts he has "traveled through time and space", but again, it is simply not confirmed. For now, I think the two are the same person.
I've heard that theory. Not sure if I believe it or not. Sounds plausible, though.