I appreciate the offer, but no thank you.
Zakath15 responded w/the above; the first and MOST POLITE AND APPROPRIATE RESPONSE IF YOU DONT' WANT TO GO.
First and foremost, I am polite and mannerly. A fine line (.1um) after that, I'm a sarcastic, vengeful bastard. There's a virtual library I could write about "Organized Religion and The Weak Human Psyche" but we won't go there right now.
Let me say this as an older, more wordly person; you're screwed.
This guy/gal will be insulted if you refuse his offer. He will feel slighted, and in the future will treat you like a subhuman. I hope for your sake he's not your boss.
Accept the invite, and whether you feign joy or sadness,he/she/it will forever view you as a "succesful convert from sin." Of course, you'll be expected to then live the life "of a good Christian" and quote the bible and get married and have 234 children within the first year, of course.
And should you faill to live up to these unreasonable (by sane standards) standards, you'll be an outcast...just like i fyou refused the invite in the first place.
Your course of action is clear. POLITELY refuse the invite. AND WHEN HE/SHE/IT questions you as to what your grounds for refusing the invite are, you do the ultimate "ball in your court" and say:
You know, a TRUE Christian wouldn't ask me why I chose to worship God in my own way and not yours.
Say that. ^^^ It messes them up every time.