This is something that - I think - requires individual discernment; as the teaching of many denominations is either that they and they alone are The Church and others are heretical (and often treated as worse that atheist), or else they adhere to an anti-Christian ecumenicism.
Here is something to think about:
If you believe in Mere Christianity, then perhaps you ought to support, with prayer and worldly effort, other Mere Christians who are successful in evangelical and mission work? - even if/ when you regard their type of Christianity as sub-optimal.
The kind of Mere Christianity which wins converts in most of the world at present is Protestant, evangelical - do you regard these denominations as really Christian; and if so, do you support them - or do you perhaps, instead, spend most of your effort denigrating them and distinguishing yourself from them?
You cannot be indifferent: If you are not for them, then you are against them.