Could you help an old alter boy, Father

I think it simply means that the demon has knowledge of things that Regan could not know. Possibly it simply has access to Karras' memories.

That scene gets more attention in the book. Karras is losing his faith, and the tramp disgusts him so much, he has another moment of doubting God.

The demon also spoke in Karras's mother's voice, as well as a host of others. Merrin was wise to instruct him to avoid conversing with it.

Although I can't exactly remember what part of the film this happens in, I always took it that the demon was trying to weaken him by guilt shaming him, because of his turning his back on the tramp who asked him earlier on. It's the same tactic the demon uses by pretending to be his dead mother. It's all about getting inside Karras's head.