The traditional forms of Christianity - the main denominations or churches, are - it seems to me - extremely brittle. They are all or nothing, unreformable without starting a collapse leading-to destruction - they lead to, and seem to require, hardness among those who administer them.
Unfortunately, this characteristic brittle-hardness and imposition upon the individual is, I firmly believe, intrinsically and irreducibly anti-Christian.
So that, while we might say that overall and on average, many or most traditionalist churches have been - for significant periods, net-Christian (overall Christian, more than 50% Christian...); they have also had times, places and persons when they were certainly net-anti-Christian; because, whatever their practices, however 'correct' their behaviours; their motivations have been sinful.
This applies in different ways to Calvinist protestantism, Eastern Orthodoxy, Anglicanism... but the clearest example is the Roman Catholic Church.
Old style Roman Catholicism - as seen in Ireland or Spain up to the 1970s; or in parts of Britain with Irish communities such as Glasgow and Liverpool - was a sustainable religion; but it had plenty of oppressive and dead-ly aspects. Very 'school dinners'. It had strong elements of arbitrary authoritarianism, oppression, harshness...
The hope among some sincere and real Christians seems to have been that Roman Catholicism could be sweetened, warmed, made more fluid and organic... but the attempt to do this was instantly subverted into dilution, apostasy, and assimilation to secular leftist politics. So, in the late-1960s there was Vatican II implemented, the replacement of beautiful Latin with bureaucratic vernacular, an explosion of Marxism among religious orders, a release and facilitation of sexual abuse from among the radical priests; and overall less practice, less belief, less Christianity.
I feel that with the RCC it is all or nothing - to be viable it needs to be authoritarian, heavy-handed, and anti-individual; and any attempt to reform the undesirable aspects will just smash it.
This has also been the case for the other big Christian churches; although the decline has tended to be less immediately obvious than with Roman Catholics. The Church of England and international Anglican communion seems to have been unaware that it was held together mainly by the Book of Common Prayer and Authorised Version of the Bible. This was highly prescriptive, and the language was becoming less comprehensible... so from the 1920s it began incrementally destroying these unifying texts until nothing now holds the church together but its secular bureaucratic structure.
The big Nonconformist churches - such as Presbytarians, Methodists and Baptists - were known for their strict ethics, plain living, detachment from hazardous activities such as gambling, drinking, and mass media. The life was narrow and rather colourless and lacking in culture... but when the restrictions were eased, the nonconformist churches crumbled to dust; either disappearing or becoming de facto social clubs.
If this argument is true, which I believe it is (and Christian traditionalists agree that there is no such thing as reform, and that all pretended reforms are actually destruction); then it is a real problem if there is going to be no going back to the old practices. If, that is, there cannot be a return to any form of traditional religion of any kind; then Christianity is doomed...
This cannot might mean three things. 1. It might mean that, for various socio-political reasons, it is extremely improbable that the mainstream churches can, in practice, recover from their current shrunken and corrupted states... that there is just not enough to build-on; the baddies out-number and out-rank the goodies several-fold.
2. It might mean that we are now so deeply sinful that we have ceased even to want salvation, that we have come (en masse) actively to desire our own damnation and those of others; and we have entered the end times; and the world will move-into the state of general depravity that precedes the end of the earth and the second coming of Jesus.
Or, 3. which is my understanding, that 'cannot be a return' may be because God's destiny for mankind is linear and sequential, and the traditional era of a hard-brittle Christian religion is now left-behind us; and our only options are either to stay in the current phase of progressive collapse and corruption or else to move forward to some unprecedented form of Christianity - something real, devout, primary, central to life and living; and something that has never previously been seen on this earth.
To traditionalists, who identify Christianity with one or another Church, and for whom Christianity outwith that Church (or some Church) is an oxymoron (self-refuting nonsense) this is just a version of 2. - that we moderns are too sinful to want what we need.
My conviction is that - for many modern people in many modern situations - the Christian possibilities within a Church is worse than the possibilities outside. But more accurate is to say that churches are, or ought to be, secondary; most people cannot and should not depend on them primarily. When a Christianly-helpful church is available and possible, then support it; when not, get on with being a Christian on your own.
I think the best possibilities of a Christian future will depend on those who take responsibility for their own Christian faith. But the 'mechanism' by which this will take effect (if it does) will not be socio-political; but will instead be spiritual-mystical - and observable in terms of remarkable 'coincidences' and unforeseen 'luck' - due to God arranging things invisibly, in the background.
If we, each as an individual, really are pushing in the direction that divine destiny wishes us to proceed; then this is exactly what will happen. We can be sure of it.