The second main point of this sermon is "to be pure in heart, is the certain and only way to attain to this blessedness. 'This blessedness refers to the first point which states that it is a truly blessed thing to the soul of man to see God.
The aspect I will focus on, hence drawing your focus to, is introduced with "But the purity of heart with respect to sin, that may be obtained in this life, consists in the following things". The following things are listed below with some quotes.
1. 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 ... But he 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."
2. It implies godly sorrow for sin. "The pure heart has not only respect to that spiritual filthiness that is present to abhor it and shun it, but it has also respect to past sin. The consideration of that grieves it; it causes shame and sorrow to think that it ever rejoiced in such defilement, that it ever was so abominable as to love it and feed upon it. Every transgression leaves a filth behind it upon the soul, and this remaining filth occasions pain to the renewed and purified heart. 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."
3. 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 principal seal, the throne of the heart, that was formerly possessed by corruption, is now purged, and filthiness does now as it were only possess the inferior and exterior parts of the soul."
4. The heart that is pure will be continually endeavoring to cleanse itself from all remaining filthiness. "So he who is pure in heart will never suffer himself to live in any sin. If he be overtaken in a fault he will return and cleanse himself again by repentance, and reformation, and a more earnest care that he may avoid that sin for the future."
5. The heart is said to be pure, especially with respect to its cleanness from, and opposition to, the lust of uncleanness. "This kind of wickedness we find to be more especially called uncleanness and filthiness in Scripture. It brings a peculiar turpitude upon the soul, and defiles the temple of God."
We must not think to excuse ourselves by saying that it is God’s work, that we cannot purify our own hearts. For though it be God’s work in one sense, yet it is equally our work in another ... If you do not engage in this work yourselves, and purify your own hearts, they never will be pure. If you do not get a pure heart, the blame of it will be laid to your own backwardness. The unclean soul hates to be purified. It is opposite to its nature. There is a great deal of self-denial in it. But be content to contradict the nature and bent of your own heart, that it may be purified. However grating it may be to you at first, yet consider how blessed the issue will be. Though the road be a little rough in the beginning, yet it will grow pleasanter and pleasanter, till at last it will infallibly lead to that lightsome and glorious country, the inhabitants of which do see and converse with God.