I have, since I became a Christian, usually said that I was a Mere Christian in a modified sense of that used by C.S Lewis - which is that I am a Christian first, and the denomination and specific church comes second and third; or, that there are several/many Christian denominations which I consider validly Christian, the choice between which is more a matter of lifestyle rather than salvation.
I still believe this - yet as time goes by the conviction is tempered by a concern that many/ most ostensibly Christian denominations and specific churches do net-harm to the average Christian member (I am thinking particularly of the 'liberal' Christian denominations).
On top of this, I have experienced an increasing sense of the importance of marriage and family as the major secondary aspect of Christian life.
Consequently, I felt a mounting sense of horror in contemplating the demographic composition of most self-styled Christian denominations and churches, and particularly the statistic of sub-replacement sized family size among so many supposedly Christian groups.
I have come to regard above-replacement fertility as a necessary (but not sufficient) aspect of any Christian denomination or grouping - this 'demand' is non-negotiable except where fertility was being actively prevented.
(Not above-replacement fertility of each and every specific church member - of course not! But of the group as a whole. So that the unmarried, married couples without children or with small families, and celibates are on the average compensated-for by the fertility of the rest.)
So that any denomination or national church which displays sub-replacement fertility, I would regard as defective and - on the whole - not Christian; and it does not matter a jot what their theoretical creed or confession may be. Theoretical creeds are just documents - and anybody can say they believe anything they choose; but if people are not living by the creed, then the creed is irrelevant.
And vice versa - when people are living as Christians (so far as I can tell) then they are Christians, by a broad-brush definition; and the micro-detail of creed and confession is as irrelevant as it is when (as more usual) the creed is 'orthodox' but being ignored.
Anyway, at the end of all this I perceive that the concept of Mere Christianity is - in practice - just a good idea.
I think it is sometimes very helpful to new converts such as myself, that is intellectuals outwith any active church domain; it can get us across the line to be Christians without becoming 'hung-up' on the specifics of 'which church?'
But then the problems arise.
With mainstream institutional Christianity in such a profoundly corrupt state; the wrong choice of denomination or church may be a disaster to the convert - being more likely to deconvert them than building them in faith.
(Aside, I find the sheer venom of inter-Christian infighting among serious Christians to be profoundly dismaying. Of course they are not fighting and killing one another; but in our weakness I hoped for goodwill and cooperation among real Christians - but it does not take much provocation for the sniping and backbiting and mutual damnation to begin. And the relish with which this sport is prosecuted is the worst aspect of the business.)
The new convert may find that among Christian churches that are real and alive (therefore not ruled-out by the evidence of being de facto anti-marriage and anti-family), there may not be any to which one can readily make a wholehearted commitment.
Or, at least, making oneself commit to a specific actual church - when working alone and without help from enjoying the process of church-going - can be a long haul.
And in the meantime we are on our own.
I can be as Merely-Christian as I want, and so presumably can other people - but as a mass movement it lacks both traction and a basis for cohesion.
So it seems that Mere Christianity has individual value and mainly as a temporary expedient - while trying to sort-out a specific church or a denomination - although the 'temporary' phase may turn out to be long-lasting, perhaps even 'permanent'.