The heart is central to this because in the Bible the heart stands for the seat, source, reservoir, and instigator of our thoughts, attitudes, desires, character, and motivation. It is synonymous with our modern use of "mind," since the mind is where we hold knowledge, attitudes, motivations, affections, desires, likes, and dislikes.
Jesus says in Matthew 5:8, "Blessed are the pure in heart, for they shall see God." Obviously, the quality of the heart is the issue in this beatitude. Proverbs 4:23 reads, "Keep your heart with all diligence, for out of it spring the issues of life." Our Father directly addresses the book of Proverbs to His sons (Proverbs 1:7). It assumes our hearts have been purified by His initial cleansing, that we have received His Spirit, and are in the process of sanctification and going on to perfection. Ezekiel explains this process:
Then I will sprinkle clean water on you, and you shall be clean; I will cleanse you from all your filthiness and from all your idols. I will give you a new heart and put a new spirit within you; I will take the heart of stone out of your flesh and give you a heart of flesh. I will put My Spirit within you and cause you to walk in My statutes, and you will keep My judgments and do them. (Ezekiel 36:25-27)
This does not all take place in an instant. It is a process, and as we have all discovered from Scripture and our own experience since baptism, human nature is still very much alive within us (Romans 7:13-25). Paul confirms this in Galatians 5:17, "For the flesh lusts against the Spirit, and the Spirit against the flesh; and these are contrary to one another, so that you cannot do the things that you wish."
Human nature, the law of sin within us, is always seeking to pull us again into the defilement of sin, seeking to destroy our hope of sharing life with the holy God. That is why God counsels us in Proverbs 4:23 to keep—that is, guard, preserve, and maintain—our heart. It is very easy to become defiled by lapsing back to old habits. In stark reality, Romans 8:7 and Jeremiah 17:9 show why: "Because the carnal mind is enmity against God; for it is not subject to the law of God, nor indeed can be." "The heart is deceitful above all things, and desperately wicked; who can know it?" The normal human mind deceitfully convinces each person that they are good and love God, men, and law. But the reality is just the opposite: It is at war with God and men, and hates God's holy, righteous, and spiritual law. It loves itself and its desires far more than anything else. It is this deceitful, self-centered enmity that exerts constant influence, pulling us into the defilement of sin.
Jesus preaches on this in Matthew 15:16-20:
So Jesus said, "Are you also still without understanding? Do you not yet understand that whatever enters the mouth goes into the stomach and is eliminated? But those things which proceed out of the mouth come from the heart, and they defile a man. For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies. These are the things which defile a man, but to eat with unwashed hands does not defile a man."
It is sin that defiles holiness. In terms of character, of being in the image of God, sin defiles, pollutes, contaminates, or blurs the reflection of God in us. "If we say we have no sin, we deceive ourselves, and the truth is not in us" (I John 1:8).
John W. Ritenbaugh
The Beatitudes, Part 6: The Pure in Heart