The whole theme of his book has been an exposition of Romans 8:13. In the ESV it says, "For if you live according to the flesh you will die, but if by the Spirit you put to death the deeds of the body, you will live." Owen's tagline is "Be killing sin or it will be killing you."
For the rest of the book, Owen shows how people try to kill sin the wrong way, using their own penances and rituals. Instead, he wants them to leave human effort and come to Jesus to be changed.
There is only so many ways you can say this concept, but nothing is more important in reaching salvation and being sanctified in the Holy Spirit.
Basically, Owen says that people try to mortify sin, but they do it the wrong way.
He reached a point in his treatise where he gives the example of Peter's speech in Acts 2. In verses 29-36, Peter explains how David died but looked forward to the resurrection.
God loved David, but without his help, David was a wicked man indeed. In himself, there was nothing good. But because of his faith in God's promise to send an eternal king on his throne, David died with the hope of living again and with the hope of freedom from sin.
"This Jesus God raised up, and of that we all are witnesses." God raised his eternal Son from the dead. Why did Jesus die? Because we are so hopelessly lost in our sins. On our own, we will not come to God for salvation but always look to whatever we can do. Jesus loved us so much that he lived the only perfect human life and then died under God's wrath that humans deserve. Since he was perfect, he could not stay dead, and since he was the perfect sacrifice, we are not enamored with our sins anymore.
"Let all the house of Israel therefore know for certain that God has made him both Lord and Christ, this Jesus whom you crucified."
The penitent ones heard what Peter said and were "cut to the heart." If I was among the crowd, I would think, "The only good man on the planet died so that I could be free from sin. What must I do to repay him?"
This is what the folks asked. Did Peter say to go wash seven times in the Jordan? Did he say complete five years of a discipleship course? Did he tell them to pray five times a day to some heavenly sponsor?
No, he simply said, "Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit. 39 For the promise is for you and for your children and for all who are far off, everyone whom the Lord our God calls to himself."
All they needed to come to Jesus. The Holy Spirit would baptize them, and they could finally be fit to enter God's presence. And if a man of the household came to the Lord, his children and wife and servants could also be baptized and be considered holy because of him. Simply coming to Jesus makes no only myself clean, but also makes my whole family clean by association (though not necessarily individually saved. They are still considered holy along with me and we must pray for Jesus to truly lead their hearts.)
The bottom line: quit trying to find holiness apart from simply resting in Jesus. He will make you holy. The people you love are also holy because of you in their lives, so treat them as inside the faith and let Jesus change them. Remember to forgive them as the Lord forgave you. What you did to Jesus is way worse than what they've done to you or anyone else. Trust the Lord to change their hearts and simply love them in the process.
I close with this 2nd Chapter of Acts song: