if by "agnostic" you truly mean someone who doesn't quite know one way or the other, then simply be curious about their religion. by being curious, you don't have to buy it, but you give them the "satisfaction" of being interested. and why not? if you're really "agnostic" then you are not threatened by the possibility of finding out the truth wherever it might lie.
if, though, you are saying you are an atheist, then, depending on how conservative they are, that has the usual effect of not going over very well with christians. for those kinds of christians who believe that jesus is their personal savior, they tend to take that shit personally too. you dis the existence of jesus, they dis you.
sticky wicket, to be sure. but, like others have said, you are at the age of adulthood in most countries, so it is time for you to start acting like an adult and take responsibility for your beliefs. that means you learn how to tolerate others and you take heat for your beliefs and let the criticism roll off your back...or learn from it.