Owen brings so much content to the table concerning the nature and power of sin in the life of the believer, it is like trying to drink from a fire hose. His thought process, logic, and ability to synthesize Biblical information is beyond comparison. The book is actually a compilation of three books: On the Mortification of Sin in Believers, Of Temptation: The Nature and Power of It, and Indwelling Sin.
In the first book, On the Mortification of Sin in Believers, Owen makes the distinction, from the very beginning, that no unbeliever can mortify sin, just as Paul shows in Romans 8:5-8. Sin is only mortified by the Spirit. Unbelievers are without the Spirit, therefore, whatever good they may have attained to is a self-righteousness they have wrought within themselves. And as Isaiah 64:6 states of works done to try and please God outside of God and the righteousness He provides (namely through Christ), “all our righteous acts are like filthy rags,” and are offensive even to God. “Everything that does not come from faith is sin,” and therefore no unbeliever has a part in true mortification. They may deceive themselves in this manner, thinking they have truly reformed themselves, just as the Pharisees thought. But this is not the case. Owen goes on to describe what mortification is not, by going into detail about the very nature of how men think they have mortified a sin by “morally reforming themselves,” when really the sin still lies in wait until the opportune time to strike, or morphs into another kind of sin (i.e. lust turning to pride or covetousness). Then he goes on to describe what mortification actually is, and that it cannot and will not occur outside of the supernatural work of the Holy Spirit through the work of Jesus Christ to strike at the root of the sin. Owen also makes the point that if you are not daily mortifying sin, even though you may think you are simply lying still and being complacent, you are actually being anesthetized by the sin and under attack, where it gains a greater foothold in your life, making it even more difficult to deal with. Then finally, at the end of the book, Owen states that all he has written up until that point was meant to prepare for the actual work of mortifying sin, and then gives directions pertaining to the work of mortification itself.
In the second book, Of Temptation: The Nature and Power of It, he goes into great detail about what “Entering into Temptation” actually means. He uses the Scripture where Jesus is telling the disciples in the Garden of Gethsemane to be watchful and pray that they may not enter into temptation. Many suppose this means simply to avoid temptation, but Owen describes that being tempted and entering into temptation are two entirely different things. Jesus was tempted but never entered into temptation. Once a person enters into temptation, meaning they make a conscious decision to flirt or play with a sin “at a distance” (so they suppose), it is like a quick sand that pulls you down further than you ever wanted to go, moving you to do things you never thought you could do, bringing about ruin and misery in your life.
The third book, Indwelling Sin, is basically Owen’s unpacking of Romans 7, where Paul lays out the great war going on within him between his sin (his old nature) and the Spirit (that is, the new nature created in Him by the Spirit). The great conflict within all regenerate believers in the Gospel can be summed up in the Latin phrase Simul iustus et pecator – that is, we are simultaneously justified yet sinful at the same time. And there is an unimaginable war going on within us as a result, sin fighting vigorously and deceitfully for sovereignty in our hearts over against the sovereignty of the Holy Spirit in our hearts. He shows how the heart is the seat of the affections we have within us. And for sin to reign in our hearts is to control our affections. Owen dissects the various ways in which sin attacks every facet of our being, to keep us in bondage, in order to carry out offenses against God.
In all of these books, Owen strives to show with Scripture, reason, and extensive arguments, our absolute inability to do any spiritual good that matters in God’s eyes. We our poor, wicked souls, that are in need of Gospel/Cross healing, and that is our only hope even. We are utterly dependent upon God to work within us that which is holy and pleasing in his sight, that He might change our affections, desires, wills, to line up with that of Christ’s. God has given us the means of grace (Scripture, prayer, fellowship, accountability, communion, baptism) in order that we may come to Him and be healed by Him alone. We must make use of the means in order that God may work in us and pursue Him and the supernatural change He makes within us, on His time frame and will. He is sovereign, we are not, in every way. The hope of the Christian alone, in becoming conformed to the image of Christ, is that God is sovereign to change hearts through the cross of Christ, and is indeed willing to do just that. We are on His time frame though. We do not have a wishful hope that God would move in us, but a sure promise that, “He who began a good work in you will bring it to completion at the day of Jesus Christ.” (Philippians 1:6) And we must fight with all the tools and resources God has given us, to have this change done within us by the Holy Spirit alone.