"God has always existed."

 

"Muslims think they are right, but I know Christianity really is right."

 

This is where your opponent sets up a rule to use for their argument, but then wants to make an exception to this rule for no good reason. They want their version of things to be "special". You can point out they are not being consistent by doing this.

 

In the first example, theists often argue that things can't have always existed, they must have had a beginning. They then argue that god must have made the universe. But then you can say to them that god also can't have always existed, so where did he come from? And they say god doesn't need a beginning. This is blatantly ignoring the rule they themselves set up for the argument, with no justification. If the rule doesn't apply to everything, then it may not apply to the universe either, removing the need for god in the first place.

 

In the second example, when theists are challenge to explain why other people believe just as strongly in other religions, they will simply say theirs is special. They set up a rule where believing in a certain god doesn't automatically mean that god is real, so that other religions can be false. But then they try and avoid the same rule, by just announcing that their god, religion or book is special and that it really is true. This is totally invalid, not least because every other religion can make exactly the same argument.