When christian relatives hear that I am atheist?

Most likely you are presenting it with the purpose of "ELICITING" such things. Or you are interpreting their concern for you as fear and shock. The most hardcore Christians I know (and I know some of the really loonie ones, too) aren't shocked in the least when they hear someone is an atheist.

Because they have had in drummed into them by those who stand to gain that anyone who isn't part of their "gang" is bad in some way. You know that there really is a god (their god, of course!), and you are either just playing up, or you don't want to take responsibility for your actions.

Because they were raised with the misguided belief that atheists are immoral, going to burn in hell, etc... So they are somewhat afraid of you and your wicked ways, somewhat concerned for the fate of your immortal soul, etc...

All you can really do is go about your life and try your best to ignore it. Either they will realize that you're still exactly the same as you were before they found out you were an atheist or else they will remain hateful people who you are better off not spending time with anyway.

They're terrified that you may be correct.

Maybe they're just sad that they have a strong Christian family for God, which many people love strongly including me. But they'll get over it.

They don't want you to go to Hell.

They don't believe it. They mind is not alterable.

They can't believe anyone could be that stupid.

Because you always seek a stranger's opinion before theirs (as in this case) and so they think you are a coward at heart.

It's kind of like satan walking in the room.