A conversation with my friend Luke got me thinking. He had a prof at Fuller who suggested that there are three things one must believe to be a Christian: 1. that people are sinful. 2. the trinity 3. the divinity of Jesus.
That got me thinking about that question further. The first point is something that is particularly important in that if one does not affirm sin, then there is no need for redemption and this God does not have to actively pursue us nor do we need God. This reduces the importance of our contingency. Moreover, I think sin is a pretty common sense concept - the hard part is accepting that sin is serious and deserves punishment. It is a hard world where folk start in the negative instead at even keel.
The trinity is a more ambiguous subject. There are parts of the idea behind trinity that is very important to the ethos of Christianity such as the central importance of community over individualism and the limits of human rationality and comprehension. It might be possible to suggest trinity in such a way that Jesus does not have to be divine: adoptionism without the idea that God left Jesus but died with him instead or a doscetisim where we take seriously the nonreductive physicalism of the soul.
The divinity of Jesus is one of the more controversial. I don’t see much need to claim Jesus as fully divine but I am on the non literal side of interpretation for much of Scripture.
This leads me to my own thoughts: sin and the character of God as salvific by Jesus death and resurrection are the only ultimately necessary componeta of Christianity.
Note: that is not the definition of who is saved just who is a Christian. I do not view those as equivalent sets of people.