I think that unfortunately we will all encounter someone that feels their beliefs are correct and should be held by everyone else. I would explain to them that while you can respect their viewpoint and understand that this is the way they feel others should live, that you feel differently. Politely tell them that as you respect them, you ask that they do the same by not being aggressive with their faith. If they insist on continuing the behavior, it may just be best to remove yourself from the situation. That is the most mature way to handle someone that cannot respect boundaries. Thank them for their time and move one.

I think a good way to turn aside someone else's religious fervor is to say, I appreciate your concern for me, and I know you will be praying for me, too, and that is a wonderful thing for you to do. At this point, I think you need to back off verbally and just let God do His work in me. I know you don't want to mess up His plan by forcing yourself on me when it's time to just let Him do His own work.

If that doesn't work--and it should--then ask them if they have ever read Ecclesiastes 3:7. If the haven't, tell them that you think you may be there on God's behalf to lead them to that verse.

Verse 1: There is a time for everything,
and a season for every activity under heaven:
Verse 7: a time to tear and a time to mend,
a time to be silent and a time to speak.

If that doesn't work, then tell them you don't believe they are hearing from God at all and walk away

Politely? Not sure if I'm the right guy if you want a polite response. I'll tell them politely no thanks once, beyond that they're the ones being impolite and I have no trouble responding in kind. If you're not looking for a confrontation, just raise your voice a little and say firmly something along the lines of "I said thanks, but not thanks". If you're in the mood for something a bit more festive however, it's allways fun to point out that christianity is essentially a belief in a benevolent zombie who's his own father had himself killed to save us from the sins we hadn't yet committed. It also usualy creates a good show if you start comparing jesus to the easter bunny.

If you absolutely must be polite, perhaps look them in the eye for 2 seconds, switch to a lower pitch and say something along the lines of "look, I know you mean well and I appreciate your effort, but I'm not interested". Something like that, use pitch and body language to stress that you are being genuine.

Reach for your cell phone like it's vibrating and excuse yourself from the situation. Thank them for their viewpoint and then walk away. Or just outright tell them that you don't share their view and do not discuss your religious beliefs openly with strangers.