Ooh, this one I want to give much more time to answering then I have...

Many different aspects, some weaker, some stronger, that come together enough that I feel very confident in my confidence.

And not every belief within Christianity is something I'd say I'm sure I'm right, or it's vital for every Christian to believe/agree.

But my main logical foundation would be something like this:

• 'God', who created existing physical reality, exists.
• the Bible, being a special testimony and writings of people's interaction with God, is trustworthy
• Jesus is God come into the world as a human, and he died, and came alive again
• a Christian is someone who follows Jesus and accepts forgiveness through Jesus and knows and worships God through Jesus.

Those are pretty much axioms/dogma, and it would certainly take more time than I have now to describe why and how I think they are true. But I feel confident to say anybody who rejects those has no business calling their beliefs 'Christian'.

(Essentially. ...there are a handful of subtleties depending on how you interpret what I've written.)

From those axioms there are a bunch of things that I think are essential for healthy Christian thinking & living, and a bunch more that I value highly, and/or think correct, but I'll still respect you as a Christian if you disagree.

And for the most part, this core foundation (including many of the corollaries) is shared by pretty much every Christian group that considers Jesus their lord rather than some other person or 'humanity' or 'human logic'. Although the different groups will often discuss and express it in very different terms and perspectives.

For the question above of what is 'sin', Christians will talk of obedience, broken relationship with God, rebellion, evil spiritual influence, corruption, decay, human nature, foolishness... all things that have connection, and some become just different starting points to talk about the same thing.

Part of the problem is whether you start with a concept and choose a word (e.g. sin) for it; or start with a word and ask what other people (e.g. the Bible authors, Christian history etc) have meant by it.

And of course on the way people will twist and abuse it all sorts for their own ends, which is a major reason we always go back to the Bible for authority.

All that said - I think there is a strong argument from the Bible, backed up (for myself) by my experience of my relationship with God and (for others) by common church teaching, that the core meaning of sin is being not-loving, not-kind, selfish, hateful, evil, towards people and towards God. I.e., being a dick. That judgement is recognising publicly between wrong (being a dick) and right (not being a dick) and rewarding/punishing appropriately. And that most of how sin and righteousness is discussed and characterised beyond that, is framework around that, taking into account long term effects, the reality of God, and so on.

I don't expect every Christian to agree that my way there of analysing it is the best, or even helpful, but I would expect most if not all - if they fit my axioms of Christianity above - if they really study and consider their beliefs and the Bible, to come to a similar conclusion as part of their understanding.

Phew! I wonder if I made any terrible mistakes as I wrote. (And on my phone, at that!) Eh, it's the internet, you can get what you want from it and assume I'm a dick for the rest ;-p. And I'll try and answer any more questions if you like.

Oh, a final note. My description of 'judgement' seems to miss out the central thing of Christianity, that is we are saved (from our sin, and from judgement regarding it) by trusting in Jesus. That is central, (and maybe I've not written it well), but it's of key importance that that - that salvation through Jesus - is built on top of that plain meaning of right/wrong and judgement.