If you aren't swift enough, the missionaries will ask you something on the lines of whether you have found Jesus Christ. This might ask for a bit of explaining on your part, but don't be afraid of losing your thread in random ramblings. Like most religious fanatics, Mormon missionaries have a blind spot for non-sequiturs and bungled syllogisms in general, so you should get through with free association and graphic metaphors. I mean, THEY believe that the prophet Joseph Smith received the Word of God, in the United States, in the form of millennia old golden tablets written in some ancient script he could decipher effortlessly with the help of the Angel. Another way to stop the missionaries dead in their tracks would be to pose sex-related questions.
Sex always works.
Be sure to ask at least five times about polygamy and how you are sad that the current Mormon generation is no longer allowed to keep multiple wives. If you are straight, grin like a fool and sigh deeply from time to time. For obvious reasons I took another route and inquired about their take on queers. You can bring up the polygamy issue as well by mentioning how you always have had problems scheduling your three lovers. Be sure to appear as queeny as you manage without getting sick, I even succeeded letting my hand hang in a most satisfactory way.