What's a similarity between The Ark of the Covenant, The Holy Grail and a bunch of 12 year olds?
They are all locked in the Priest's basement.
This joke plays on the dark stereotype of priests engaging in inappropriate behavior with children. The setup lists revered religious objects (the Ark and Grail), making the punchline unexpectedly disturbing by connecting them to a sinister image of child abduction, creating a shocking and uncomfortable twist.