Absolution plothole?
Just watched the film (whilst reading the book) and something occured to me when John does the last-rites bit on Balthazaar.
He points out that being 'half' human, Balthazaar could be forgiven and thereby granted entrance into heaven. Afterwards, he notes that you have to ask for forgiveness to receive it...
My thought is: if Constantine knows this; why doesn't he go to a church, get absolved by the priest/other officiator, and get his free pass back into Heaven?
Colour me stupid flavoured here, since I'm not exactly well-versed on the religiosity angle, but that seems to be the way it works.
Better to have what you don't need, than need what you don't have. - my proverb.