I would ask them why.Hopefully we can have a Bible discussion afterwards.

It is their choice.

 There really isn't much you can say to them. All you can do really is  show them by example  how you live your life and how you are as a "believer". Many a person has been won over through a christians christ like conduct.

I would ask why they believed that. Then I would listen. Then I would explain my personal one on one relationship with Yeshua and ask them about their relationship with Him.

I would ask them why not, since they obviously have a number of false impressions, misunderstandings, and/or ignorance concerning God's Church, which I would help them understand better

I'd ask, "So what else is going on?"

"Correctamundo!"

Wait. 

That's what I would say

if they said I'm not a Christian.

If they said they were not a Christian

I would say,

"Congratulations!"

--

Regards,

John Popelish

I'm 73 - I already know who is or is not a Christian.

EDIT

Among my family and friends 😎

~

What would we be talking about? You need to throw us a fricken bone here.

I might  ask them if they would be interested  in seeing  what the Bible says about the subject, depending on their tone of voice.  

After years and years of friendship, a friend of mine told me she was an atheist.  

I said, "Okay."