Chas was an angel the entire time.
Seen this theory floated around before, but now I think it has to be true. I'm not sure about the comic book mythos, or the TV series. I can only go by what the movie tells us, and that's basically that when humans die, they go either to Heaven or Hell. They don't become angels, and it would be impossible to become a half-breed (as that is just an angel or a demon that is part human). So Chas was a full fledged angel at the end, but that must mean he was one the whole time.
The only problem is, it doesn't make sense either way you look at it. Some people think him acting rather childish and impressionable was just for Constantine's benefit. But then how come he was acting the same way when no one else was around (like in the taxi, and when he taunts the bouncer at Midnite's club)? Maybe he was supposed to be a rookie angel or something.
Ultimately, though, I think it does give the film more weight if we look at it this way, as it means that God was directly watching over John the entire time.