…there is nothing in God’s universe that is so utterly useless as a merely formal Christian. I mean by that, one who has the name but not the quality of a Christian. The apostle Paul describes this when he speaks of certain people ‘having a form of godliness, but denying the power thereof’. They appear to be Christian but they are not. They want to appear as Christians, but they are not functioning as Christians. They are salt without savour, light without light, if you can imagine such a thing. You can do so most easily, perhaps, when you think of the illustration of the light being hid by the bushel. If you test this by observation and experience you will have to agree that it is the simple truth. The formal Christian is a man who knows enough about Christianity to spoil the world for him; but he does not know enough about it for it to be of any positive value. He does not go with the world because he knows just enough about it to be afraid of certain things; and the people who live right in the world know that he is trying to be different and that he cannot be wholeheartedly with them. On the other hand he has no real fellowship with the Christian. He has enough ‘Christianity’ to spoil everything else, but not enough to give him real happiness, peace and joy and abundance of life…They are finally outsiders. They are more outside, in a sense, than the man who is entirely worldly and makes no claim or profession, because he at least has his own society.
– Martyn Lloyd-Jones, Studies in the Sermon on the Mount, pp. 150-151.