Marrying someone illegal?

Ok so I'm going to be honest with you. I've been with my boyfriend for almost 5 years. We've discussed marriage the last 2 and now we're ready to get married. The problem is, is that he isn't legal. We both don't want any added problems. I'm also very afraid of the outcome as far as immigration is concerned. Is there anyone out there that can give me some real honest advice or suggestions on what to do? We weren't trying to get married based on his legal status. It was for the approval and blessings of God( for you that believe in God). But I don't know what to expect or how this works. Can anyone help me? Please?