…was that they were interested in details rather than principles, that they were interested in actions rather than in motives, and that they were interested in doing rather than being. The remainder of this Sermon on the Mount is just an exposition of that. Our Lord said to them in effect, ‘You are pleased with yourselves because you do not commit adultery; but if you even look with lust in your eyes, that is adultery.’ It is the principle, not the action only, that matters; it is what you think and desire, it is the state of your heart that is important. You do not become Christian by just refraining from some actions and doing others; the Christian is a man who is in a particular relationship to God and whose supreme desire is to know Him better and to love Him more truly. That is not a part-time job, if I may so put it, it is not achieved by the religious observance of a part of Sunday; it demands all the time and attention we have. Read the lives of the great men of God and you will find that that is the principle that always emerges.
– Martyn Lloyd-Jones, Studies in the Sermon on the Mount, p. 181.