This is, for me - as a late adult convert to Christianity, one of the most important insights to make sense of world and historical Christianity.
The 'publicity material' for many or most (but not all) genuine Christian churches has tended to be exclusivist; claiming that only 'we' have the keys to salvation: that only inside the church can you be saved, and outside of this particular institution all are damned.
There have been times when I have tried to believe this of a particular church, or even to make sense of it - but honesty prevents this. It is crystal clear that real Christians, and indeed the very best Christians, have existed across many denominations, and as members of many churches and of no particular church.
Exclusivist claims by churches in relation to salvation are therefore factually false. Exclusivist claims can only be, should be, interpreted as matters of expediency or strategy - as a concession to human weakness and vacillation; perhaps necessary on average, in some circumstances and for some people.
(But it is better - preferable - when a church can function without making exclusivist claims in relation to salvation.)
Therefore, my interpretation is that in Christianity there is one mystical church but many denominations and individual institutional churches.
The one mystical church is the community of those who have repented and currently accept Christ's salvation.
(Others will join this mystical church later in life, and still more will accept Christ after death, when confronted with the reality of the situation).
So what is the legitimate purpose of the specific, institutional churches?
Their proper purpose is to sustain faith and encourage the spiritual progression - the becoming-more-divinized, that is the theosis, or sanctification - of their members.
A good church is one which supports Christian faith on average and in individuals by various means - such as teaching, sacraments, prayer, study; by structures of legitimate authority and by informal social mechanisms.
A bad church does the opposite - and tends to destroy Christian faith on average, or in particular individuals.
Therefore, each church can be judged on average, and also at the individual level. Some bad churches, which destroy Christian faith on average - such as the most of the mainstream and most powerful Liberal Christian churches within both Protestant and Catholic denominations - may nonetheless be helpful for specific individuals (for instance, those who fight these anti-Christian trends by staying in bad churches).
So, denominations and churches are not about saving us, but about sustaining us in faith and moving us further towards the status of Sons of God.
What differs in each church is the emphasis and scope. Some churches
are happy with a moderate piety, others encourage members to scale the
heights (and there are advantages and risks to each tendency). Some
churches focus on celibacy and the monastic/ religious life - others
focus on marriage and the family as the pinnacle.
The different emphases are
not a matter of indifference, they do matter. Some churches have a better emphasis than others - especially in relation to particular times and places and people.
Some churches are much more correct than others in terms of what they teach, recommend
etc. Some churches provide access and encouragement on a path to higher levels of spiritual
progression than do others. Some churches do not even claim to enable high levels of theosis, but focus instead on evangelism. Some churches have an international scope and potentially a social-political role - but with all the hazards that brings, others are tiny and local and pure.
Some churches are competent and coherent, while others are a
mess; some are mostly-uncorrupted, while others are full of fake Christians in leadership positions; some have a warmth of spirit, others are emotionally cool - or even cold.
Specific churches and denominations are very important - they can be very helpful, or else a major hindrance to spiritual progression: In sum - specific Christian churches make a huge difference (for good or ill); but none of them are crucial to individual salvation.