Create in me a clean heart, O God, and renew a right spirit within me. Cast me not away from your presence, and take not your Holy Spirit from me. Restore to me the joy of your salvation, and uphold me with a willing spirit.(Psalms 51.10–12)

Sin is more about what we have lost than what we have gained...
In this famous Psalm of repentance, David begs God to create in him a "clean heart" by restoring something. David's murderous pride and lust was the result not of gaining something (power and the sight of Bathsheba bathing, for instance) but of losing something. He had lost the joy of his salvation. The battle against sin is lost when we lose our gratitude for the cross and all that it has accomplished (Col. 1:13). Consider Christ's words to the church in Ephesus:

“ ‘I know your works, your toil and your patient endurance, and how you cannot bear with those who are evil, but have tested those who call themselves apostles and are not, and found them to be false. I know you are enduring patiently and bearing up for my name’s sake, and you have not grown weary. But I have this against you, that you have abandoned the love you had at first. Remember therefore from where you have fallen; repent, and do the works you did at first. If not, I will come to you and remove your lampstand from its place, unless you repent (Revelation 2.2–6)

No matter how hard we work for God our lampstand is in jeopardy if our work is not motivated by the joy of our salvation. If it that is lost, our first priority is to regain it.

Traditional "accountability groups" do not work...
Here's why: In most American accountability groups, men gather together (generally for no more than about 10 consecutive weeks) to recount their weaknesses and sin. This is necessary (1 John 1:5-10) but only defensive. The fight against sin is only after it has occurred. Worse yet, any "clean bill of spiritual health" exults us while sin may still exist in the heart! So goes the defensive accountability group - arriving late to the battle and never striking any definitive blows against the enemy.

These "sin-reporting" accountability groups have this as their war-time strategy:

"OK, men. I want you to head out into that battlefield and wait. Stand there and wait. Don't pull a trigger, don't launch a mortar, don't call in for reinforcements, don't swing a sword until the enemy has moved behind your lines and struck a definitive blow. Then, after this initial decimation, fight with all your might and then only at the periphery of the enemies' formation, never at the heart."

Wars are not won on these terms.

We must go on the offensive
We must go on the offensive with sin, to gain what we have lost and, for believers, every sin can be traced back to a loss of joy for our salvation. We gain that joy in at least three ways.

1) Through obedience. It is easy to think that obedience is a pathway to sanctification. Our holiness is determined by our obedience... right? In 1 John the apostle presents another, more compelling reason for obedience in the justified believer.

By this it is evident who are the children of God, and who are the children of the devil: whoever does not practice righteousness is not of God, nor is the one who does not love his brother. (1 John 3.10)

We know that we have passed out of death into life, because we love the brothers. Whoever does not love abides in death. (1 John 3.14)

Little children, let us not love in word or talk but in deed and in truth. By this we shall know that we are of the truth and reassure our heart before him; (1 John 3.18–19)

Whoever keeps his commandments abides in God, and God in him. And by this we know that he abides in us, by the Spirit whom he has given us. (1 John 3.24)

The presence of sin in our lives creates a downward spiral. As we sin, like David (1 Chron. 21:30) we "feel" condemned and our confidence (Hebrews 4:16) erodes. This does violence to the joy of our salvation. Conversely, obedience reveals the gracious presence and power of the Holy Spirit in us; a deposit guaranteeing what is to come (2 Cor. 5:5). This confirmation reinforces the joy of our salvation and kills the root of our lusts.

Beyond obedience to God's law, we must also obey the Gospel.

Then they said to him, “What must we do, to be doing the works of God?” Jesus answered them,“This is the work of God, that you believe in him whom he has sent.” (John 6.28–29)

[God will inflict] vengeance on those who do not know God and on those who do not obey the gospel of our Lord Jesus. They will suffer the punishment of eternal destruction, away from the presence of the Lord and from the glory of his might, (2 Thessalonians 1.8-9)

Obedience to the Gospel is a matter of trust. What do we trust for our salvation, our comfort, our security, our identity, our life and our joy? When the answer is "the Gospel" then we are obeying the Gospel. When we trust in anything else, either our self-righteous effort or our self-centered indulgences, we are disobeying the Gospel. This disobedience, again, diminishes our confidence and damages the joy of our salvation - a loss which opens us to more sin.

Obedience is offensive, not to gain justification (or sanctification) but to be secure in it and to have that security change our affections.

2) Through prayer. In Psalm 51, David prays for the restoration of his joy. He prays because the restoration is God-dependent. Paul's prayers for the churches are very similar (Phil. 1:9-11, Eph. 1:15-18, Eph. 3:14-19 and Col. 1:9-12). Asking God to help us see the greatness of what He has done for us and the glory of His Son is a major offensive to gain new ground in the heart. As God answers this prayer, our greatest joys are found in what we have already received in Christ and that crushes sin.

3) Through good use of the weapons God has provided for the war...
Which is our topic for next week.

For now, remember defense is not enough in the battle against sin. We must be offensive.

"Thus, our victory of mortification will be realized to a large extent as we weaken lust's presence and enticements. We must implant, promote the continual residence of, and cherish those graces that stand in direct opposition to the lust. So, for example, by the implanting and growth of humility, pride is weakened. Passion is weakened by patience, uncleanness by purity of mind and conscience, and love of this world by heavenly-mindedness. These graces of the Spirit, as they are expressed in various ways, weaken the perplexing lusts that wage their warfare against us.

Our victory will be further realized as the new man immediately springs into action, and cheerfully fights against the lust the moment it appears. We must use ever weapon available to conquer it!" - John Owen

Additional Resources
Fight at the right time and fight to win - Message from Week 3 of "Kill or Be Killed"
Fill in the Margins - offensive weapons galore
The Explusive Power of a New Affection - A very important sermon by Thomas Chalmers
Discussion Questions for Week 3 (Chapters 5 and 6 of Mortification of Sin)