The pure in heart are continually endeavoring to cleanse themselves from sin. The new nature of a Christian, even though there are pockets of sin remaining, is so contrary to the old man that it desires to be wholly sanctified. According to Jonathan Edwards:
“Blessed are the pure in heart: for they shall see God.” (Matthew 5:8)
It is the heart that God looks at. However fair and pure an outside there may be, that may be very pleasing to men, yet if there be not purity of heart, the man is not at all the more acceptable to God. 1 Sam. 16:7, “But the Lord said unto Samuel, Look not on his countenance, or on the height of his stature; because I have refused him: for the Lord seeth not as man seeth; for man looketh on the outward appearance, but the Lord looketh on the heart.” If men outwardly behave well and speak well, yet it is not accepted without trying and weighing the heart. Pro. 16:2, “All the ways of man are clean in his own eyes, but the Lord weigheth the spirits.” It is the spirit which is the subject of this blessedness of seeing God, and therefore the qualities of the spirit, and not so much those of the outward man, are regarded.
The heart is said to be pure with respect to the filthiness from which it is pure. Sin is the greatest filthiness. There is nothing that can so defile and render so abominable. It is that which has an infinite abominableness in it. And indeed it is the only spiritual defilement. There is nothing else that can defile the soul. Now there are none in this life who are pure from sin in such a sense that there is no remainder, no mixture of sin. Pro. 20:9, “Who can say, I have made my heart clean, I am pure from my sin?” So that if this were the requisite qualification, none of the children of men would ever come to see God. . . .
But the purity of heart with respect to sin, that may be obtained in this life, consists in the following:
It implies that the soul sees the filthiness that there is in sin, and accordingly abhors it. Sin, that is so filthy in itself, is become so sensibly to the man whose heart is pure. He sees its odiousness and deformity, and it is become nauseous to him. . . .
[H]e who has become pure in heart hates sin. He has antipathy to it. He does not love to be near it. If he sees any of it hanging about him, he abhors himself for it. He seems filthy to himself. He is a burden to himself. He abhors the very sight of it, and shuns the appearance of it. If he sees sin in others, it is a very unpleasant sight to him. As sin, and as committed against God, it is grievous and uncomfortable to him wherever he discovers it. It is because his heart is changed, and God has given him a pure nature.
It implies godly sorrow for sin. … By godly sorrow, the heart exerts itself against the filthiness of past sins, and does, as it were, endeavor to cast it off, and purge itself from it.
It implies that sin is mortified in the heart, so that it is free from the reigning power and dominion of it. Though the heart is not perfectly free from all sin, yet a freedom is begun. Before, spiritual filth had the possession of the heart, corruption had the entire government of the soul, every faculty was so wholly defiled by it, that all its acts were filthy, and only filthy, the heart was entirely enslaved to sin.
But now the power of sin is broken, the strong bands by which it was tied and fastened to the heart are in a great measure loosed, so that corruption has no longer the possession and government of the heart as before. (“The Pure in Heart Blessed”)