"A Christian can generally be known by his very appearance. The man who really believes in the holiness of God, and who knows his own sinfulness and the blackness of his own heart, the man who believes in the judgment of God and the possibility of hell and torment, the man who really believes that he himself is so vile and helpless that nothing but the coming of the Son of God from heaven to earth, and His going to the bitter shame and agony and cruelty of the cross could ever save him, and reconcile him to God‑‑this man is going to show all that in his whole personality. He is a man who is bound to give the impression of meekness. He is bound to be humble. Our Lord reminds us here that if a man is not humble, we are to be very wary of him. He can put on a kind of sheep's clothing, but that is not true humility, that is not true meekness. And if a man's doctrine is wrong, it will generally show itself at this point. He will be affable and pleasant, he will appeal to the natural man, and to the things that are physical and ; but he will not give the impression of being a man who has seen himself as a hell‑bound sinner, and who has been saved by the grace of God alone"
~Martyn Lloyd-Jones (1899–1981)
Taken from Studies in the Sermon on the Mount [Grand Rapids: Eerdmans Pub. Co., 1977] pp. 258‑259.