This is a prototypical example of why I love the Puritans in general, and Thomas Watson in particular. He takes the phrase, ‘I kept myself from my iniquity,’ and probes the depth of the human heart with us, calling us out for our idolatry and besetting sin. Be aware that Thomas Watson was no legalist, he was a preacher of the gospel, yet he was not afraid to press the application of a passage to the very core issues concerning sin. Besetting sin is a lust-affair. Here are some excerpts:

There is usually one sin that is the favorite—the sin
which the heart is most fond of. A godly man will not
indulge his darling sin: “I kept myself from my iniquity.”
(Psalm 18:23). “I will not indulge the sin to which the
bias of my heart more naturally inclines…

Question: How shall we know what our beloved sin is?

Answer 1: The sin which a man does not love to have
reproved—is the darling sin..Men can be content to have other sins
reproved—but if the minister puts his finger on the
sore, and touches this sin—their hearts begin to burn
in malice against him!

Answer 2: The sin on which the thoughts run most, is
the darling sin…Examine what sin
runs most in your mind, what sin is first in your
thoughts and greets you in the morning—that is
your predominant sin.

Answer 3: The sin which has most power over us, and
most easily leads us captive—is the one beloved by the
soul. There are some sins which a man can better resist.
If they come for entertainment, he can more easily put
them off. But the bosom sin comes as a suitor, and he
cannot deny it—but is overcome by it…

Answer 4: The sin which men most defend, is the
beloved sin…The sin we advocate and
dispute for, is the besetting sin. The sin which we
plead for, and perhaps wrest Scripture to justify it
—that is the sin which lies nearest the heart.

Answer 5: The sin which a man finds most difficulty in
giving up, is the endeared sin.

The besetting sin is, of all others, most dangerous.
As Samson’s strength lay in his hair—so the strength
of sin, lies in this beloved sin. This is like a poison
striking the heart, which brings death. A godly man
will lay the axe of repentance to this sin and hew it
down! He will sacrifice this Isaac; he will pluck out
this right eye—so that he may see better to go to
heaven.

Now here’s my summary and gospel applications for each point:

Summary: Your pet sin, the nicotine for the smoking soul, is the one:
1. You don’t want to be called out on
2. You think about the most
3. That has the most power over you, that you find hardest to resist
4. That you will do anything to justify
5. That you won’t give up

What do you do?
1. Hear the gospel of Jesus Christ, repent, and believe:

• Romans 8:1 ¶ There is therefore now no condemnation for those who are in Christ Jesus. 2 For the law of the Spirit of life has set you free in Christ Jesus from the law of sin and death. 3 For God has done what the law, weakened by the flesh, could not do. By sending his own Son in the likeness of sinful flesh and for sin, he condemned sin in the flesh, 4 in order that the righteous requirement of the law might be fulfilled in us, who walk not according to the flesh but according to the Spirit.
• 1. When our Lord and Master Jesus Christ said, “Repent” (Mt 4:17), he willed the entire life of believers to be one of repentance (Martin Luther, 95 Theses).

2. Turn your thoughts upon him and his gospel constantly:

• Romans 8:6 To set the mind on the flesh is death, but to set the mind on the Spirit is life and peace.

3. Consider how much more Jesus suffered for your sin than you will for turning away from it:

• Hebrews 12:3 ¶ Consider him who endured from sinners such hostility against himself, so that you may not grow weary or fainthearted. 4 In your struggle against sin you have not yet resisted to the point of shedding your blood.

4. Consider that in justifying your sin, you are neglecting the One who truly justifies the sinner:

• Luke 16:15 And he said to them, “You are those who justify yourselves before men, but God knows your hearts. For what is exalted among men is an abomination in the sight of God.

5. Consider how your laying aside of sin pales in comparison to the resolve of Christ, who laid aside his glory:

• Philippians 2:5 Have this mind among yourselves, which is yours in Christ Jesus, 6 who, though he was in the form of God, did not count equality with God a thing to be grasped, 7 but made himself nothing, taking the form of a servant, being born in the likeness of men. And being found in human form, 8 he humbled himself by becoming obedient to the point of death, even death on a cross.