How, then, is this mystery cleared up? By what method, or in what way, have the sanctified become blest with a nature which makes them meet for the ineffable presence of God? By what process does the evil tree become good, so that its fruit is wholesome and acceptable? Obviously, we cannot here supply the full answer to these questions, or we should be anticipating too much that we desire to bring out in later chapters. But we will endeavor to now indicate, at least, the direction in which and the lines along which this great mystery is cleared—lines which most assuredly would never have entered our hearts and minds to so much as conceive; but which once they are viewed by anointed eyes, are seen to be Divine and satisfying. The Lord graciously assist us to steer clear of the rocks of error and guide us into the clear and refreshing waters of the truth.
...We may clearly perceive that it is they who are truly sanctified and holy, who are the most deeply sensible of the root of corruption which still remains within them, and which is ever springing up and producing that which defiles them; and therefore do they greatly bewail their pollutions, as that which is most dishonoring to God and most disturbing to their own peace; and earnestly do they endeavour after the mortification of it. A remarkable corroboration is found in the fact that the most godly and holy have been the very ones who most strongly affirmed their sinfulness and most loudly bewailed the same. It was one whom God Himself declared to be a ‘perfect (sincere) and an upright man, one that feareth God, and escheweth evil’ (Job 1:8) who declared ‘Behold, I am vile’ (40:4). It was one ‘greatly beloved’ of God (Dan. 10:19), who acknowledged ‘my comeliness was turned in me into corruption’ (10:8). It was he who was caught up to the third heaven and then returned again to earth who moaned, ‘O wretched man that I am! who shall deliver me from the body of this death?’ (Rom. 7:24).
Every Christian, then, has a ‘pure’ heart in the particulars given above. But every Christian does not have a ‘clean’ heart (Ps. 51:10). That which pollutes the heart of a Christian is unjudged sin. Whenever sin is allowed by us, communion with God is broken, and pollution can only be removed, and communion restored, by genuine repentance—a condemning of ourselves, a mourning over the sin, and unsparing confession of the same, accompanied by a fervent desire and sincere resolution not to be overtaken by it again. The willing allowance and indulgence of any known sin cannot exist with a clean heart. Rightly, then, did John Owen say of repentance: ‘It is as necessary unto the continuance of spiritual life, as faith itself.’ After the repentance and confession, there must be a fresh (and constant) recourse unto that Fountain which has been ‘opened for sin and for uncleanness,’ a fresh application by faith of the cleansing blood of Christ: pleading its merits and efficacy before God.
...We have sought to answer the questions at the close of the fifth chapter. We have met every demand of the law in the person of our Surety. We are made meet for the inheritance of the saints in light, because all the value of Christ's cleansing blood is reckoned to our account. We are capacitated to draw nigh unto God now, because the Holy Spirit has communicated to us the very nature of Christ Himself. By faith we may regard ourselves as holy in Christ. By regeneration we have received a ‘pure heart:’ proof of which is, we hate all impurity, although there is still that in us which delights in nothing else. We are to maintain communion with God by cleansing our own hearts (Ps. 73:13), and that, through constant mortification, and the daily and unsparing judgment of all known sin in and from us. - A.W. Pink, The Doctrine of Sanctification