During the Main Quest "Mysterious Ways" you will go to Uzhitz and part of that quest will be to talk to Father Godwin. You should make a save before talking to him - just in case.
During that conversation you will have three options to convince him to break his vow. You need to fail convincing him because if you succeed he will not invite you to the tavern. To get the most out of it you should choose to Threat him as that will have you brawl with him. The safest way would be to turn up dirty and use Charisma to fail.
You should make another save before talking to him at the tavern, see warning below
At the tavern you need to accept drinking with him. The achievement will unlock after a few scenes.
WARNING: Accepting to drink with Father Godwin will void the "Virgin" achievement. Reload the last save and choose not to drink get back on track with that one.