I assume you are talking about the cellar door involved in Broke Steel side quest "Holy Water". You can open this door 5 different ways. 1) Asking Gerard to let you in. This requires you to have 600 rad points. 2) You can also ask Gerard to let you in while wearing the Ghoul Mask. 3) Pick the lock (Very Hard). 4) Simply kill Gerard and loot the key from his body. 5) If you don't want to kill him, pickpocket the key off of his person.