Disclaimer: I don't own them so please don't sue :D.
Edward doesn't believe in God, he has based his entire life philosophy on that, but every time Roy spreads his legs and pounds into him he believes he is getting closer to heaven. It's because of this that Roy is the only person to ever hear Ed scream God's name.
"Oh, God."
"God, please!"
"More, oh God more!."
And every time afterwards Roy laughs at the sheer irony of it all, causing Ed send a sharp glare his way. He proceeds to ask him the reason for his laughter, but Roy just strokes the boy's cheek and says, "Nothing."
Of course in some morbid way Roy relishes in this. He know of Ed's rampant atheism, so he has started to truly believe that he is God in Edward's eyes, and that every night Ed prays to his Lord to fuck him harder.
Roy has gone so far to suggest putting stain glass windows in his room. Confused, Ed asks, "What the hell do you think this is, a church?"
That question alone send Roy into a fit of laughter confusing the blonde even more.
However, it is because of that confusion and anger within him that causes Ed to keep coming back. For, behind the tightly locked doors and carefully covered windows of the Colonel's home this was Edward's church, his sanctuary from the harsh reality of the outside, and Roy was his saviour.