It is a stumbling block for potential Christians that Jesus is necessary for salvation. And, such is the essential nature of Christianity, that this is a matter which modern people find it very difficult to understand - or, at least, they find it hard to understand as something good.
Quite reasonably, people find it unacceptable that people should be pressured (indeed blackmailed) into becoming Christians by the 'or else' kind of threat represented by the idea of being 'sent to hell' if you don't. There is also a worry about the billions of people (past and present) who either know nothing about Jesus and Christianity or know only some kind of biased/ prejudiced/ selective version - or else are unable to understand it (due, for example, to being babies or young children, mentally handicapped or mentally ill/ brain damaged).
So, it seems unreasonably, and indeed wickedly, restrictive for salvation to be confined only to those who know, understand and accept Jesus. Or else eternal torment in hell...
But the other side of the coin is that salvation should not (indeed cannot), be forced upon anyone - Christianity is an opt-in kind of religion; and that opt-in needs to be conscious, deliberate, a kind of 'informed consent'. How can this be the case - given the above problems of ignorance, misinformation, incompetence, lack-of-capacity... How could everybody be given a fair chance to opt-in?
One answer might be to consider what happened with the resurrection of Jesus; and that he promised resurrection to everybody. Death is the separation of soul/ spirit from body - the body dies, and the spirit remains. Yet the spirit alone is a maimed thing, hardly self-aware, unfree, 'demented' - and this was widely recognised in ancient religions, especially before Christ; where the realm of the dead (Hades, Sheol etc) was a place of barely-sentient spirits. Not a place of torment, but a place where we lost our-selves - forever (unless there was reincarnation).
So, the process of resurrection has at least two aspects: the first is that spirit and body are reunited, we become sentient again, we regain our souls, our selves...
The other part of resurrection is the spiritual process of being re-born to eternal life. This involves a positive, conscious, deliberate choice - because what this entails is allying ourselves permanently with God's plan, his goal of a reality based on Love. Heaven is this world based on Love between persons.
We cannot be coerced to love (else it is not love) - and indeed we would not want to dwell in Heaven if we did not want to live in this world of loving relations - more exactly we cannot live in this world of loving relations if we do not, ourselves, love.
But to be able to live in such a world is not something that we can accomplish for ourselves - it is, indeed, the gift of Jesus. This is why there is no other way than by him. To live in heaven we must believe in, have trust in, Jesus - must surrender our-selves to him so we can be remade fit for heaven.
Such absolute, trusting surrender is only rational if we believe that Jesus loves us. In other words, we must believe-in Jesus - his power, goodness, love - in order to surrender utterly to him; in order to be able to participate in eternal life in Heaven.
For this to be universally available to all men and women, at all times in history and today, regardless of circumstances and place - then it must be something which occurs (or at least can occur) after death. That is; everybody must be brought, after death, to a situation in which he or she makes a fully-informed choice, with understanding of the consequences. This is 'judgment' - and it is our personal choice (although Jesus was responsible for setting-up the choice).
Those who did not (for whatever reason) repent during mortal life are able to repent after mortal death, in this fashion. That is, they can choose whether to accept the gift of Christ, or not.
Hell is what happens to people who choose not - Hell is the people who choose not to live by love, who choose not to trust Christ.
But why specifically Jesus Christ, why must we believe in him personally? Now that the system is set-up - couldn't Christ's role be discarded?
My understanding is that Life, including eternal life, is ultimately personal - not abstract. I regard this as one of the essential aspects of Christianity - because Love requires persons.
It was the work of Jesus as a person to enable us to be saved from permanent death (severance of spirit and body) by repentance, and to be resurrected to eternal life. It was (it seems) necessary for Jesus as a person to go-through what he did (incarnation, birth, life, death and resurrection), and to do so by choice, for us to receive the benefits. It was necessary for Jesus to do this in order that we (that is all men and women) can follow the same path.
Why exactly this should be so is another matter - but that it is so is central to the Christian story.
Anyway, my take-home-message here is that belief in Jesus is indeed necessary to salvation; because being saved entails a surrender of our self to Jesus; and without belief (faith, trust, love) we will not allow ourselves to be saved.
Furthermore, all men and women have been and will be presented with this choice to believe-in Jesus Christ or not - and this choice in full clarity of consciousness and sufficient comprehension of the implications - regardless of their earthly circumstances. This situation is something that is always (sooner or later) made possible and arranged by divine action and intervention.