It is interesting to try and understand the essence of 'Christianity' - the single main thing that Jesus did, if you like - and when you do, it seems that there are actually quite a wide range of answers.
My own (Fourth Gospel derived) idea is that Jesus brought the new possibility of eternal resurrected life in Heaven to those who followed him.
Others regard the coming of Christ in terms of setting up a new religion - and then participating in the prescribed activities of that religion. Or a changed relationship between Man and God. Or provision of a source of guidance for (this mortal) life. Others focus on a change in 'reality' in the totality of the universe - an 'evolutionary' view. Others take a morality-focused view; which see immorality/ 'sin' as The Problem, and Jesus offering a solution. There are also other ideas - a surprisingly large number!
But if my own understanding of Christianity as "following Jesus to Heaven" is accepted (as a thought experiment, if nothing more) then it is interesting to consider what happened during the years of Jesus's ministry when a Jew or a Roman Pagan decided to 'convert', to become a spiritual follower of Jesus - to consider what this meant in terms of their pre-existing religion.
My understanding is that - initially - a belief in Jesus was understood in terms of an addition to what was already believed. The Jew added a belief in Jesus to his pre-existing religion, and the Roman likewise.
In other words, the new thing about becoming a follower of Jesus was the expectation ('hope') of eternal resurrected life in Heaven - to come after this mortal life and after death; and that this resurrection was to be attained by following Jesus.
'Following' involved having faith in Jesus (in his being the Son of God, thus divine - therefore able to do what he claimed) - and 'faith' meant something like loving and trusting him.
I think this - specifically about the Jews - is what Jesus meant in those passages of the Gospels where he implied that no aspect of 'being a Jew' needed to be changed in order to become one of his followers.
It also fits with the miracles of faith; where the miracle happens in someone who 'believes-on' Jesus - specifically, personally; without regard to the nature of his specific religious life - which might be Jewish, Samaritan, or any type of Pagan.
This idea of 'Christianity' as pre-existing religion-plus, fits with the observation of people who seemed to be (to to believe themselves to be) Christians and Jews, or (presumably) Christians and Samaritans, or Christians and Roman or Greek Pagans.
It happens because the original Christianity was actually composed of "pre-existing religion"... and then 'adding Jesus'.
This idea of Jesus as 'an addition' to religion is quite different from - almost the opposite of - 'syncretic' ideas of a religion formed from combining aspects of "Christianity" (as it later became) and "some other religion".
The idea is instead that Christianity has an essence - which is the following of Jesus to resurrection - and this essence can be added to almost any other "religion".
But of course, adding Jesus does not leave the pre-existing religion untouched, unchanged! Far from it! In the first place, the Christian idea of death as followed by resurrection (for those who believe-on Jesus) must displace whatever description of death was given by the pre-existent religion.
So the expectation of Heaven needs to replace Sheol, Hades, paradise, reincarnation, annihilation or whatever was previously expected.
Furthermore, adding-Jesus inevitably works-back on the pre-existing religion.
In other words, the expectation of immortal life in Heaven affects the understanding of mortal life on earth - affects it in innumerable ways.
I suppose that this was the basis for the development of the various Christian churches - these are the various consequences of the expectation of Heaven, on Man's understanding of life on earth.
And the churches vary because the order and priority of these changes strikes people differently. Since the changes in mortal life are secondary consequences of the primary reality of resurrection; there will often be disagreement as to which ought to come first, which ought to be most enforced.
But our situation here-and-now, in 2022, is that of no pre-existing religion. "Following-Jesus" cannot easily or obviously be added to Zero - not in the way Jesus could be added to Judaism or Paganism.
Atheism is the - increasingly mandatory - basis of all serious social life and discourse. Religion is everywhere subordinated to ideology - and that ideology is top-down, imposed, and evil.
Our rituals and rules are secular (i.e. Satanic in nature and by intent) - not divinely-attributed.
Jesus is not believed in his promises because he 'cannot be' divine, because 'the divine' is seen as untrue, mistaken, a lie - and indeed impossible.
Resurrection and eternal life are seen as sheerly incoherent in a materialist world where spirit and the soul are seen as merely mythical, pathological or manipulative.
Such is our situation. Men of 2000 years ago (and much more recently) were able to understand what Jesus meant easily and quickly. They could become 'Christians' (followers of Christ) almost instantaneously; simply by understanding that Jesus offered something more than their existing religion, and by experiencing the spiritual conviction (faith) that this offer of resurrection was real and possible.
The offer still stands, and can still (in principle) instantaneously be accepted - and then the expectation of Heaven can still begin its inevitable (but unpredictable, because so wide-ranging) working-back to transform a Man's pre-existing convictions...
Yet the Good News of Jesus Christ cannot nowadays simply add-onto and re-shape existing religious convictions because there are none, and even the basis for religion is destroyed and replaced by a secular, materialist, leftist ideology*.
(An ideology which is in-actuality a mostly-covert Satanism - not neutral but evil.)
The potential follower-of-Jesus ("convert") must also choose to reject many foundational modern metaphysical assumptions concerning the nature of reality; and choose instead to adopt beliefs within-which Christianity makes sense, and can do its work.
And against the background of evil materialistic nihilism - this must indeed be a conscious choice; and (because the world of institutions and rules opposes it) this conscious choice must be personal - that is, individually-motivated.
The essence of Christianity remains the same as ever - what makes a huge and adverse difference is that Modern Man is deeply damaged by this-worldly materialism, a nihilism that is deep and habitual, and by a tacit-and-denied allegiance to the evil agenda of Satan.
*Note: This is why I regard it as a misleading error to call the global, mainstream modern ideology of 'leftism' (or political correctness, or 'woke') "a religion" as so many of those who oppose it do. Despite some superficial resemblances, the leftist ideology is not a religion, it is instead anti-religion - the negation of religion. This is proved by the fact that it cannot (like Judaism, Greek or Roman Paganism etc) be added-to by Jesus, to make someone a real Christian. Precisely because leftist-ideology is Not a religion; if you "add Jesus" to leftist ideology you merely get a fake-Christianity. You get that Christianized-leftism of the kind propagated by the leaders of the major Christian churches and denominations.