I love using this example to that whole "Jesus is the ONLY way to heaven" schtick.
First, quote Joshua 24:15 -
"And if it seem evil unto you to serve the LORD, choose you this day whom ye will serve; whether the gods which your fathers served that were on the other side of the flood, or the gods of the Amorites, in whose land ye dwell: but as for me and my house, we will serve the LORD."
Emphasize the "my house." Ask them if they think Joshua is talking about the building he lives in, or if he is talking about his family, servants, slaves, etc.
Then quote John 14:2 -
"In my Father's house are many mansions: if it were not so, I would have told you. I go to prepare a place for you."
Ask if they think Jesus is talking about a building, or God's "family" (refer back to Joshua). Point out that a "house" normally does NOT contain additional buildings - but a family can have many houses, all of which can be different. Point out that a clear interpretation of this verse (by using Joshua 24:15 as precedent) is that Jesus is saying that in God's family there are many "mansions" (or religions), and that Jesus is preparing a NEW mansion (or religion) for his followers. Then emphasize how Jesus himself said, "If it were NOT SO, I would have told you." Thus, Christianity is simply one "mansion" for God's "house".
Watching their heads explode is hilarious. :)