According to C.S. Lewis, this is a piece of advice from a senior demon about how to cause a man to lose his faith in Christ:
Whichever he adopts, your task will be the same. Let him begin by treating [cause A] or [cause B] as a part of his religion. Then let him […] come to regard it as the most important part. Then quietly and gradually nurse him on to the stage at which the religion becomes merely part of the ‘cause’, in which Christianity is valued chiefly because of the excellent arguments it can produce in favour of [cause A] or [cause B]. The attitude which you want to guard against is that in which temporal affairs are treated primarily as material for obedience. Once you have made the World an end, and faith a means, you have almost won your man, and it makes very little difference what kind of worldly end he is pursuing.
When I hear some of the things that come out of the mouths of professing Christians, I am often reminded of this passage. Those baffling statements are very often explained by the speaker thinking that Christianity is ‘all about’ something other than Christ; even, especially perhaps, when what it is supposed to be ‘all about’ is something that Christians should in fact be doing. If you think that Christianity is ‘all about’ including the marginalised, or bringing justice to the earth, or care and compassion, or giving society cohesion, or providing a moral framework, then you are badly on the wrong track. Those are all good things, but neither individually nor collectively are they an end to which faith is a means.