If you blew up Megaton then you can't get to the White Light Monastery.
You're screwed. You csn't get in there because you're a raging asshole.
All jokes aside, the Holy Water Quest is the only way in, and you automatically fail that quest [Although it doesn't say so, but the quest is never offered] if you blow the crap out of Megaton. So, sorry, but no religious quacks for you. :P