Ask them if they have the truth. Then ask them how many times their doctrines (truths) have changed. Ask them again if they now have the truth.
If they say they do have the truth, ask them what they will do when the next change is made.
Point out that they will therefore believe anything they are told, which is no different to any other religion, and you're not that mindless or gullible to base your life's beliefs on ever shifting sands, thank you.
Splash.