People who really want something always give some evidence of that fact. People who really desire something with the whole of their being do not sit down, passively waiting for it to come. And that applies to us in this matter.... The person who is truly hungering and thirsting after righteousness obviously avoids everything that is opposed to such a righteousness. I cannot obtain it myself, but I can refrain from doing things that are obviously opposed to it. I can never make myself like Jesus Christ, but I can stop walking in the gutters of life. That is a part of hungering and thirsting.
Let us subdivide that. There are certain things in this life that are patently opposed to God and His righteousness. There is no question about that at all. We know they are bad; we know they are harmful; we know they are sinful. I say that to hunger and thirst after righteousness means avoiding such things just as we would avoid the very plague itself. If we know there is an infection in a house, we avoid that house. We segregate the patient who has a fever, because it is infectious, and obviously we avoid such persons. The same is equally true in the spiritual realm.
But it does not stop at that. I suggest that if we are truly hungering and thirsting after righteousness we shall not only avoid things that we know to be bad or harmful, we shall even avoid things that tend to dull or take the edge off our spiritual appetites. There are so many things like that, things that are quite harmless in themselves and which are perfectly legitimate. Yet if you find that you are spending too much of your time with them, and that you desire the things of God less, you must avoid them.... I think it is a commonsense argument.
D. Martyn Lloyd Jones: Studies in the Sermon on the Mount, i, pp. 89-90