First you need to believe that Jesus was divine - otherwise he could not deliver on his promises.
To believe Jesus was divine you need to believe in the divine, and further that the ultimate divine is personal.
How could someone get to believe this? Well, if you trusted the Bible, or the authority of a church, then you might believe on that basis - but why would anyone trust such things unless they already believed in divinity, and that Jesus was divine?
I take it that these are matters we must come to know for ourselves, we must experience them. We must experience God, and experience that God is a person, and experience that Jesus (who is, of course, still alive - if he really is divine) was divine.
So, a religious experience is necessary - and this means that we need to believe that a religious experience can be real, and decisive.
Most modern people don't believe this; they have a prior assumption that religious experiences are delusions, that can be explained away as some kind of mistake, illness, deception or wish-fulfilment.
So, even to get to knowing by personal religious experience that Jesus was divine requires overturning a lot of contrary basic assumptions of the modern world. And this means recognising that these are indeed assumptions - and are not any kind of evidence... once we have recognised that the exclusion of Jesus's divinity is entirely a matter of prior assumptions, really - the first step of the battle is won.
But there is a second step - which is that we need to want what the divine Jesus offers us: Life Everlasting. And to want it we must understand that the Life Everlasting Jesus offers has certain features:
1. It is attained via death. We must first die before we can live forever - as happened to Jesus himself.
2. Eternal Life is as a resurrected person; that is we have an indestructible body. Many people believe is a life beyond death as some kind of spirit, or without a self; but what Jesus offers is for his followers (those who love him) to live forever as a person with a body.
3. This life eternal is a life in 'community', with other followers of Jesus; it is a life in some kind of 'place' called Heaven.
Now - the second step to being a Christian is that we must want this Life Everlasting that Jesus offers us. And it seems pretty clear that some people do not want it, or anything like it.
But how can we know, personally, by our own experience, that Jesus did make this offer, and the nature of the offer?
We might read the Fourth Gospel and experience its truth - this is what I did. But suppose we didn't have access to the Fourth Gospel, or only in a fake translation? What then?
We would then have to receive this information by direct revelation from Jesus himself - because there would be no alternative.
Is this likely? Well, yes it is likely - it is in fact certain.
Because if it is true that Jesus is a living, active God; then he will be able (some way or another) to do whatever is necessary for those who believe in him. Presumably, exactly how the knowledge is transmitted or shared will depend on the person and the situation.
So, there are two basic ways of Not being a Christian - one is not to believe in the reality of what Jesus offers: to regard the whole thing as a fraud. The other is to believe in the reality of Life Everlasting, but not to want it.