Maybe if the church gave up on tracting and 1950s door-to-door peddling then this wouldn't have been an issue instead of realizing that missionaries aren't supposed to wear beards/goatees.
You should never open the door unexpectedly - only expectedly, to people you expect... or can see through a window or peep hole. Never open door blindly.
That's why we have security doors at every entryway into our home. They are very strong metal, though in a nice design and decorator designs, that will allow you to see out, but not let anyone in unless you unlock it. They could shoot you through it, but they can't come in. I only open it for people I know and am expecting - usually, our neighbor the vet, or the pest control guy.
This is a great reason to use one of my standard door-blockers:
(The missionaries and other Mormons used to pound loudly, late at night and did not stop banging, until I yelled at them to stop.)
"Stop banging, or I'll call the police!"
In the daytime, I yell through the door:
"I don't know you, and we don't open our door to anyone we don't know."
In our driveway, our front yard, or out in public, if the Missionaries accost me, I say, "I do not talk to strangers!" I look the other way, and take out my cell phone, as I'm hurrying away.
They probably aren't all bad, but I'm not going to take the risk of finding out who is and who isn't.