Because you asked the question I have to share this joke:
(it's a visual joke so just go with it)
Why are Jehovah's Witnesses all flat chested?
(repeated pushing motion, chest high) "Get off my porch! Get off my porch!"

I politely tell them I'm not interested and send them on their way. I once had a roommate who opened the door to a couple of guys with bibles, and was like, "WHAT? THERE IS NO GOD! GTFO of here" etc.

I was about 20 years old, and thought it was hilarious.

Here in NOVA the kids and I watch for the first sign of spring. The little mormon boys out in pairs cruising the hood.

I say very calmly, first let me discuss with you Wicca, Druidism, Hellenistic paganism and of course the church of Satan since all of them are trying to convonce me to believe in their God. Do many gods, do little time. Frankly, with all due respect to you personally, your religion is just as fucked up. Thank you for your time.

Answer the door naked

Every human has the right to express and to preach their belief system. After all, what the object would be if you don't let the whole world know that you are of the X kind? Respectfully explain to them that you have chosen a different path to Destinus.

I usually tell them that there is a sign directly outside the building stating no door to door salesmen. If they persist, the manager will charge them with trespass.

About once a year I would get a visit from some Sky Fairy addicts, not sure of the particular 'brand' of 'dogma drugs' they were peddling. The man was a huge guy and his partner was black woman. When they ring the bell I simply walk to the door...look them straight in the eye...and close it. No muss...no fuss...no drama. That leaves them to stand there fumbling with their papers. I'm not going to listen to their juvenile answers when pressed about some grand plan they use to keep the conversation going round and round in circles. I know this because when I was younger I would engage them. These days my time is more valuable to me than arguing with weak minds that have lost their critical thinking skills.

I like to teach them about the Bible. But for some reason they quit coming.

I normally use them or the front door bell

But in the spirit of the question, my front door seems to be difficult to find so if someone did actually get to my front door they've probably earned a coffee lol

A black cat in one hand, and a butcher knife in the other. I have a pentacle to wear, but sometimes they catch me off guard. I have tattoos all over my limbs, so I look scary enough without black clothing...besides I also have a white cat.

We have a sign by our door, specifically telling them that we do not welcome proselytizing. It explains that we have studied the texts and come to our own conclusions and we don't need their help. That usually does the trick. If that fails or the sign gets blocked somehow (it happens), I simply reiterate what is on the sign. It is quick, simple, and there are no arguments from them. I see no need to be unfriendly or slam doors, although answering the door in some provocative manner would be fun.

Greet them with a smile and explain to them that I am not interested and bid them a good day. I see no reason to be mean or rude. They are clueless and have been convinced that they are doing a good thing.

If I have the time and energy (I am disabled) I will tell them I don't believe in God (honorific capitalization used only when speaking to someone who believes, in fact or in theory), but I will listen to their statement and even debate with them in a friendly manner as long as they do not continue to proselytize. Eventually they go away, hopefully a little wiser.

This seems to work...

If you are referring to religious door-knockers, the FSM emblem on my door prevents some of this nonsense.

I love it!!! My mother was a witness and my older brother was a born again. If I have the time, I engage them and counterpoint every word they say.

I am an atheist please come in and I will tell you why your faith is untrue and without reason or logic.

One time, I had a couple of Witnesses show up to my place. One of my roommates invited them in just to be friendly. Five minutes later there was another knock at the door. It was a young girl selling magazines. My roomates and I quickly invited her in, and then conspired to slowly excuse ourselves from the room one-by-one. So we left the three door knockers there talking to each other until they decided it had become awkward enough to leave.

I would like to thank everyone who has posted to this thread of thought. It has provided me with time to laugh, it has also showed me that I am not as creative as I would like to be. Thanks!

Although 'door knockers' are blind to the principle, they nevertheless demonstrate lack of respect for the validity of others' beliefs with the assumption that whatever those beliefs might be, that they are automatically inferior to the theology they propagate. If you don't accept the notion that your 'message' or system of belief HAS TO BE superior to any and all others you encounter at various doors, why are you even there? Hostile reactions to door knocking, I believe, are motivated by the fundamental insult that 'you need to get right' and we're here to correct you. When time permits, I enjoy their visits because of being able to play in 'their ballpark'. They are usually unaccustomed to encounters with people familiar with the Bible and history surrounding what it contains. Most often when they decide it's time to leave they approach the door in backwards fashion, grinning and promising to send 'the big guns' (elders) who they believe to be better equipped to deal with my reasoning. It can be great sport!

I've had a few and it depends on who they are. The last was a couple of old church ladies bringing bibles from across the street (I live across the street from one of the dozens of churches in town) I politely invited them in, smiled, and explained that I am not a superstitionalist and place no value on religion because I care to much about truth and being able to distinguish between what is real (supported by facts) and things that are not testable and can not be demonstrated to be honest or true. I then explained that because all religions are based on faith (belief without evidence asserted as truth to accept any faith based claim as demonstrable truth would be dishonest and I value truth and honesty more then any desire to join a group that distorts the definition of "Truth" to include silly things a person wishes to believe without any supporting evidence to demonstrate any value of truth. I then politely and kindly sited a few atrocities their bible endorses including Jephthah killing and burning his daughter so god can enjoy the smell of her burning flesh, how their bible commands them to kill me (Exodus 22:19 NAB) etc. I ended their failed attempt to have me join their church by noting, "You are both clearly nice and good people and you really don't want to talk about your bible because I've read it several times and could spend hours pointing out contradictions and atrocities in your bible. You seam happy with your belief structure and I really don't want to take your happiness away. I thanked them for coming over and they thanked me for my time.

During other events I was not as kind. My favorite encounter was waiting in the bleachers (supervising a large group of kids) with the Campus priest at a Christening for a National Guard helicopter. The aircraft was hours late so the priest and I had hours to talk. He asked me why I would not call him father and I meticulous explained (with a long series of foundational arguments) why religion and churches were evil because (among a list of biblical (BS) horror stories) they teach youth not to question ridiculous assertions leaving those who accept faith based claims to be conditioned to accept nonsense and be more vulnerable to accept lies that could hurt them and our society. I used many biblical stories to illustrate my points and he agreed with all my examples. By the end of our talk he sadly agreed that religion and churches were evil and asked me what else he could do. I felt sorry for him because he was a nice guy and he had limited marketable skills. His eyes were filled with tears so I directed him to "The Clergy Project" Months later he was still peddling his fantasy book and guiding sanctioned prayers but I am still there an I haven't seen him in years. . . Not sure what he is doing now.

There are many stories but this is already too long.

I don't get them any more as I live in sheltered accommodation, my granny used to let them in make them tea and cakes and nod in all the right places and when they had gone say -"I have done my bit for the day they left some other poor soul in peace whilst i had them here

I show them my Satanic Temple membership card. Then I hiss at them.

Not invited, not wanted = an unopened door!