Because, in fact, no one has ever asked me, and I don’t choose to mention otherwise.
Being as this office is heavily predominated by Christians (the magazine has a Bible quote on the spine, FCOL) I’ve found it’s better to be quiet about my beliefs. I get my job done, I enjoy the people I work with, and I don’t talk about religion.
You know shit, jackass. Shame on you for assuming.
Aargh.
(A note: I could have taken it as a compliment given the behaviour of persons such as Polycarp, Tris and Jodi et al. Frankly, I don’t think the person was thinking that way at all.)
Somebody just walked up to you and said
“HEY, You, yeah you! I know you’re a christian.”?
The context was a little more complicated than that.
The gentleman in question is leaving the company, and I wished him good luck.
I have forgotten the rest of the context, but basically it was “I know you’re a Christian so I’ll see you later.”
Just got under my skin.
You should have replied “See me again? - that depends where you’re headed” 
It helps to shake 'em up a bit sometimes.
Or you could’ve shot him a “L’chaim”.
or, “Right. Blessings of the Lady on you too.”
“I know you’re a Christian so I’ll see you later.”
Response: “Yes, when the Lord Mithras comes to collect the souls of Christians to burn then in his mighty bellows, then I will see you as you pass into flame and smoke and I watch… laughing.”
Or something like that. 
Sister,
I don’t get to decide who is a Christian, as far as that goes, but thanks for thinking highly of me, and putting me is such exhalted company.
I will look forward to seeing you later, too! Then we can sit down with your former coworker and explain the hurt he threw out, when he intended to give encouragement. He will feel safe enough by then not to be threatened if it goes against his theology that the Lord loves you too. We will all get a big laugh out of it.
Tris