Puritan writers like John Owen are well-known for teaching Christians to pursue holiness and kill sin. This is helpful, but Puritans like Owen also recognized that this can also lead to a serious misunderstanding on the part of Christians. In the pursuit of holiness, Christians are tempted to dwell too much on their own sins and forget the kindness of their Heavenly Father.
Think through the logic. We know that God hates sin. And although we fight sin in ourselves, no matter how hard we try, we will always find a lamentable degree of remaining sin on this side of glorification. Therefore, God must be perpetually angry with me, right?
You can see how this type of thinking can lead to legalism and to harsh thoughts about God.
So how can God be utterly for us and utterly against our sin at the same time? The answer is of course rooted in the gospel, the unmerited grace we are given in Christ’s perfect life, death, and resurrection. The answer is likewise rooted in the work of the Holy Spirit. Theologian B. B. Warfield made this connection in his study of Romans 7 ("the horror of indwelling sin") and Romans 8 ("the glory of the indwelling Spirit"). Warfield said this in a sermon on these chapters:
The Christian life on earth is a conflict with sin. And therein is the dreadfulness of our situation on earth displayed. But we are not left to fight the battle alone. The Christian life is a conflict of God — not of us — with sin. And therein is the joy and glory of our situation on earth manifested. As sinners we are in terrible plight. As the servants of God, fighting His battle, we are in glorious case. [Faith and Life (London, 1916), 202].
Here’s the key. Fighting sin is not God-against-us. Fighting sin is ultimately God’s battle, a battle that we have been caught up into. We fight with God in sanctification because no one is more engaged in our holiness than God is. No one cares more about our personal holiness than God does, a point made obvious by the fact that the Holy Spirit dwells within us.
The point Warfield makes is an important one. Our pursuit of holiness can happen — we can act the miracle — because God is for us and with us and in us. We fight sin in the employment of God and his purposes. The final victory is certain. In this way Warfield both wisely calls us to action (mortification) and gives us hope-filled motivation for the battle, which is ultimately the prerogative of our loving Heavenly Father.