stijn,
so sorry to hear about your loss, buddy. sounds like your grandfather was quite a guy. your family will be remembered in my (catholic) prayers.
ryan

To the original question... Why be polite about it? Folks coming to your door to convert you to their religion don't deserve any tact or manners, just like the scum telemarketers who call during dinner. Be rude and let them know that this form of "sharing" is not acceptable. I'm a Christian, but that doesn't make t right for me (or any other religion) to bother people in their homes. Ther's a time and a place and door-to-door is neither the time or the place...