C.S Lewis's book Mere Christianity has a reasonable claim to being the most influential book of Christian apologetics of the twentieth century - but the central concept of Mere Christianity has - it seems to me - had very little influence. And I think this is understandable since the idea, once you follow through its implications, is extremely disruptive of traditional Christianity.
The advantages of Mere Christianity include:
1. Educational - Christians of different denominations can learn from each other.
2. Strategic alliance - different Christian denominations can legitimately work together to protect and advance Christian interests.
3. Individual motivation - there is genuine flexibility of choice for the Christian seeking among the denominations: he can choose that which best suits his strengths and weaknesses and offers the best chance of salvation and theosis.
But there are weaknesses as awell:
1. Mere Christianity may weaken denominations - since each is seen 'merely' as oine among many possible paths to salvation or ways of Christian life.
2. Mere Christianity may weaken faith by introducing relativistic thinking - specific do' and don'ts become reduced in force, matters of preference rather than imperatives.
3. Mere Christianity may be anti-mission in the sense that missionary work may become internal, even parasitic; different denominations focused on 'poaching' already-Christian adherents from each other, such that growth in one church is at the expense of decline in another and Christianity as a whole diminishes.
The main question, of course, is whether the concept of Mere Christianity is actually true. I think it is true, probably; but that truth is apparently not enough to make someone give their loyalty to Mere Christianity rather than to a specific denomination.
Real Christians who are Mere Christians in theory, are often denominational in practice; and those who are genuine Mere Christians, are perhaps just feebler Christians...
So the question of Mere Christianity is a crucial one.
If Mere Christianity is true, then ignoring it will certainly lead to disaster; since Christianity will continue to lack any basis for unity.
Real Christians will therefore continue to fight among themselves, with further destruction and fragmentation; already we perceive each fragment too weak to survive alone and in long term decline.
...Until there is nothing left but individual Christian seekers united by a mystical church, disengaged from hollowed-out fake-church bureaucracies; with perhaps some small and scattered real Christian congregations each 'doing their own thing' - but no institutional church.
But if Mere Christianity is false, and yet becomes adopted as true; then it will assist the destruction of that denomination, or those denominations, which are true.
Because that which is false must be destructive of Truth.
Mere Christianity is a concept, therefore, much greater than Lewis supposed at the time he articulated it for the modern world; a time of revival and (compared with now) optimism for Christianity - assuming that the West survived the war.
Well, the West survived the war... but within a decade, Christianity began an historically unprecedented, rapid and (as yet) un-reversed decline in validity, strength and scope.
Mere Christianity, properly considered, is not an optional add-on to existing Christianity, nor an innocent basis for Christian co-operation; instead Mere Christianity has gradually been revealed as an immense and unavoidable challenge to historical Christianity.