A couple of days ago, I invited suggestions from reader about a phrase that might be used as Christianty's USP - meaning Unique Selling Point. The response was very worthwhile, with a couple of dozen suggestions, mostly very different from each other. I read and pondered them carefully.
However, in the end I found fault with all! - suggesting that summarising what was unique (and good!) about Christianity was a much more difficult thing than might be imagined --- at least when the requirement is that the answer be expressed both concisely and clearly (i.e. clearly to ignorant non-Christians).
But that still leaves the problem of how to respond to a question from someone sincere, but with a typically brief modern attention span, who asks some version of the question: What is Christianity? Or: Why should I care about Christianity?
This kind of situation represents a 'window of opportunity' - but probably only a narrow window, and one that will close very quickly.
If there is no suitably clear and concise summary possible - then perhaps the best that can be done is to say something sufficiently interesting and intriguing to invite further questioning.
Thus my suggestion of "Christianity is about what happens after you die".
I put this forward as a suggestion of something that might be said, for lack of anything better that may occur on the spur of the moment and tailrored to the asker and situation.
Such a response might provoke further interest; might stimulate further questions of a kind potentially liable to lead to the heart of the matter; and elicit enough attention to allow the answerer to get-across something substantive and potentially helpful about Christianity.
After which you can take from there; as personal understanding and specific circumstances dictate...