Are there even any mormons left around here to insult?

Na. Nothing to worry about. They have all kinds of training for missionaries, handbooks for apologetics, and most excellent professionals at organizations like F.A.R.M.S. to equip them.

How to Think of Quick Witted Comebacks

A
First and foremost, you need a quick mind. If you do not already have a smart mind, this guide is potentially useless. A good way to get around this problem is to amass a collection of as many good comebacks as you can find; you can usually modify them for different circumstances.

2
Don't listen. This really can work sometimes. For example: Person: You are such a loser! You (to your friend): Do you hear something?

d
Send their insult back at them. For example: Person: "You are such a loser." You: "If I'm a loser, then I'd hate to know what you are!" However, retaliating often results in an argument that nobody wins and makes you the lesser person. And it is usually what they want to happen.

…
Use the one line that almost never fails: "You wanna bet?" If they say yes, go up to someone and ask them if they think you are whatever the person said you are or do. For example: Person: "You are so gay!" You: "You wanna bet?" Person: "Yeah, OK!" You: (go up to someone and say): "Do you think I'm gay?" And then you can take it from there. Beware - the other person may agree and you'll end up a laughing stock.

um
Say,"It takes one to know one." And smile smugly.

3
Use the one line that almost never fails: "You wanna bet?" If they say yes, go up to someone and ask them if they think you are whatever the person said you are or do. For example: Person: "You are so gay!" You: "You wanna bet?" Person: "Yeah, OK!" You: (go up to someone and say): "Do you think I'm gay?" And then you can take it from there. Beware - the other person may agree and you'll end up a laughing stock.

11
Call names...when appropriate. Calling them ignorant or arrogant is great, so long as you don't say it in a nerdy way. Then it sounds like you're a geek who swallowed a dictionary.

#
If you are not very quick-witted, you might want to try practicing raising one of your eyebrows (or just an expression that clearly conveys you are not impressed). When someone insults you, just look at them, and raise an eyebrow. You can add a sarcastic 'O.K' or a 'Riiight', or you can just smirk/laugh at them, shake your head, or roll your eyes at them and walk away.