And he said, “What comes out of a person is what defiles him. For from within, out of the heart of man, come evil thoughts, sexual immorality, theft, murder, adultery, coveting, wickedness, deceit, sensuality, envy, slander, pride, foolishness. All these evil things come from within, and they defile a person.”
This verse in context reminds me of a passage from Paul which makes a similar point:
If with Christ you died to the elemental spirits of the world, why, as if you were still alive in the world, do you submit to regulations— 21“Do not handle, Do not taste, Do not touch” . . . These have indeed an appearance of wisdom in promoting self-made religion and asceticism and severity to the body, but they are of no value in stopping the indulgence of the flesh. (Colossians 2:20-22)
What both Paul and Jesus are saying here is that our real problem lies within us. Many of us today in our struggle against sin put too much faith on controlling what goes into us. Of course it is wise to avoid as much as we can of the daily diet of filth the world proposes for us. It is wise to choose our friends carefully, and to avoid temptation where we can. But if we only fixate on the inputs, and do nothing about the internal wiring, we should not be surprised if the outputs remain the same. Or perhaps, for a while the outputs may appear controlled, but all we are doing is setting ourselves up for an even bigger fall later on. We have all heard of people who would have made great Pharisees suddenly one day falling into the very same grievous sin that they had been condemning.
Our tendency towards sin will never be solved merely by trying to control what goes into our minds. This is as futile as hoping that fasting will remove the appetite for food. Even if our desires are controlled to some extent by such self control, we need more than that to change our hearts.
Our problem is not just with what goes into us. Some Christian men, for example, put all the responsibility for their moral purity onto women dressing chastely. The truth is that even if we took that to the extent of asking all our women to wear the Burka, the lust that lies within our hearts would not be changed.
What we need is a heart transplant.
We need Jesus to change us from within. We need to cultivate the new life that he has put in there by growing our affections for him, and for the good things he has provided for us. As we learn to love him more and more, sin will have less and less of a hold on us. As the new heart he has put inside us is cultivated, and the old one consciously destroyed by us, real lasting change will occur.
The true secret of Christian growth in sanctification lies not so much in controlling what we see, eat, and where we go (although these things are not without some value). Instead it lies in the battle that goes on inside our mind.
Do not be conformed to this world, but be transformed by the renewing of your mind. (Romans 12:2)