I admit candidly that my title is more than a little click-baitish!
However; if Christianity is considered as a religion, then it has failed - epically; given that it has had about 2000 years to succeed.
So the sensational title hides a serious point - a very serious point.
If Christianity really depends on church membership, then it has failed...
Because even after two millennia and being the official religion of several international empires; most people in the world aren't Christian church members.
The situation is even worse when it is noticed that most of the main Christian churches have been (more or less) exclusive; and through history did not recognize each other's validity as "Christian".
From the early centuries of Christianity there have been large sections of what was the Church that were excluded, divided, or destroyed - Monophysites, Arians, Pelagians, the Great Schism, the Reformation, the Nonconformists, Mormons...
Consequently; in practice and throughout history, for any particular church member, the number of "real" Christians has nearly always been very small - both numerically and as a proportion of Mankind.
And Christianity in The West now is not just shrinking in terms of church membership and attendance, but is ever more feebly motivating - makes less and less observable positive difference to how people live.
Thus we could add that if making a positive difference to how people live is supposed to be the essence of Christianity, then it has failed on that score.
If we can imagine ourselves back to the last mortal days of Jesus Christ; what happened after the ascension could hardly be termed a success! The disciples and early Christians would surely have been very disappointed indeed, at how poorly Christianity has done.
Of course the same could be said about other religions; but the difference is that Christians believe that Jesus was divine, that that he changed the whole of creation...
If that is true (and I believe that it is true: i.e. that Jesus was divine and did positively transform creation), then the limited spread of Christian churches, and their schisms and mutual hatreds and hostilities; must surely be something approaching a refutation...
I mean - looking at this world now and through the past 2000 years - does it really look as if Jesus positively-transformed creation?
All the above refutations seems to be the case... if, if, IF Christianity really is supposed to be a this-worldly and institutional religion.
If Christianity is supposed to be something that happens in this world, if becoming a Christian is a thing that must happen in this mortal life, if the definitions and membership of Christianity are in the hands of some self-identified Christian church (or even all Christian churches)...
Then Christianity must regarded as a failure.
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But, but, BUT if instead we take the Fourth Gospel seriously as as our primary source, and reason from it; then we would instead assume that Jesus changed creation in such a way that its benefits would be independent of any or all churches.
But how did Jesus change creation?
Even that is a subject of major disagreement...
Those who say that Jesus came to make better Men, better societies, and save the world - come-up against the stark fact that it hasn't worked.
But I would say that Jesus made a second creation - Heaven - and offered all Men the possibility of resurrected eternal life in that Heaven...
After Our Death.
Jesus himself died before becoming resurrected and ascending to Heaven, and so must we.
So - in an ultimate sense - it seems clear that when Jesus was doing his work of salvation, he would ensure that the situation in which any Man finds himself in this mortal life would not be allowed to prevent each individual from choosing resurrected eternal life.
To my mind, this can only mean that the final decision for or against salvation needs to happen after this mortal life.
It may happen during mortal life - any of us may choose to follow Jesus Now - but for all Men to have the chance and choice, Jesus would need to ensure that that choice does not have-to happen in mortal life.
And (being a divine creator) this is what Jesus was able to do and did - or so I presume.
By offering post-mortal resurrection into a new creation, and by making this a post-mortal choice; Jesus made "Christianity" into a society-proof faith.
All the slow and incomplete growth of churches does not ultimately matter; the apostasies and feeble faith; the schisms and intra-Christian hatreds...
These are bad; these have an effect on individual persons - but none of them have prevented the work of Jesus from happening at all times and everywhere over the past two millennia...
Every one of those who loved and desired to follow Jesus, all who would have wanted salvation - have-been and are-being given the chance to have this happen - because the choice happens after mortal life.
So, even if the religion of Christianity has failed, and seems to be headed for more and greater failure - ultimately this does not stop anyone from following Jesus Christ - even if they are ignorant or misinformed about him during their mortal lives.
The divine Jesus made sure that the evil of Men would not prevent the Good of his own work.
Good News, surely?