I regard myself as a Mere Christian - and have been trying to understand the implications; which are somewhat more complex than I had at first recognized.
To be a Mere Christian is to believe:
1. That salvation may be attained through several Christian denominations.
2. That the riches of Christ are spread across several Christian denominations.
3. That any specific and valid Christian denomination tends to certain faults - biases and incompletenesses.
4. That Mere Christianity is not a viable church; therefore that specific denominations are both desirable, and indeed necessary for there to be a viable institutional church of Christ.
Therefore, the Mere Christian nearly always has trouble in choosing (and sticking with) a Christian denomination - whether as a new convert or as a practising Christian in a world where most denominations are mostly apostate (and the situation continues to worsen).
Because, on the one hand, the Mere Christian has an ultimate indifference to denominations - such that I personally could be (and the list is not exhaustive) a conservative evangelical Anglican, an Anglo Catholic, a Lutheran, a Calvinist, Eastern Orthodox, a Roman Catholic (of the SSPX type, or Anglican Ordinariate), or a Mormon. I would not be equally happy with all of these, but certainly I could and would join and worship at any of these denominations if circumstances made it optimal.
But, on the other hand, all these denominations demand assent to more than I regard essential - and may indeed demand assent to the idea that what I regard as non-essential is essential.
So the Mere Christian could in one sense join a wide range of denominations; but in another sense, any valid denomination will almost certainly demand a wider assent than any Mere Christian could truly give.
It is a bit of a double-bind, you will agree!
Which is probably why, so far, I have stayed with my baptismal church (of England) - since more specific assent to a wider range of propositions is demanded of new converts to a denomination than cradle members of a denomination.