Why, if we're justified, should we go on to a life of holiness? What's the point? If God sees us as clothed with the righteousness of Christ and if his wrath against our sins has been exhausted at Calvary, why bother with holiness? Surely there's no need for us to bother with holiness if our sins are covered? Surely if I keep on sinning, God will keep on forgiving and getting glory?
After preaching the gospel in Romans 1-5, the apostle counteracts the charge of antinomianism he anticipates. Indeed, if you're not charged with antinomianism at some point, according to Martyn-Lloyd Jones, you're probably not preaching the gospel. In Romans 6-8, Paul fleshes out the work of Christ in fuller detail in order to counteract this charge.
The Bible speaks of sactification in two ways. I suspect that the meaning we are most familiar with is the idea of progressive sanctification, i.e. growing in grace, fruit-bearing, etc. Yet there is also a sense in which the Bible speaks of sanctification as a once for all act (indeed, when we see the word 'sanctification' translated in our bibles, it predominately means this sense). In Romans 6:1-3 Paul speaks of sanctification in this once for all sense:
What shall we say, then? Shall we go on sinning so that grace may increase? By no means! We are those who have died to sin; how can we live in it any longer? Or don't you know that all of us who were baptized into Christ Jesus were baptized into his death? (TNIV)
Baptism into Christ is a sign and seal of our baptism 'into his death' so that we no longer 'live in' sin. Throughout Romans 6, we see sin personified as something of a slave master who pays death wages (v23) to those who labour under his power. The death of Christ definitively sanctifies. It takes one from under sin's dominion and puts you in a place where you can walk in 'newness of life' (v4). Further on we read in chapter 8 1-4:
Therefore, there is now no condemnation for those who are in Christ Jesus, 2 because through Christ Jesus the law of the Spirit who gives life has set you free from the law of sin and death. 3 For what the law was powerless to do because it was weakened by the sinful nature, God did by sending his own Son in the likeness of sinful humanity to be a sin offering. And so he condemned sin in human flesh, 4 in order that the righteous requirement of the law might be fully met in us, who do not live according to the sinful nature but according to the Spirit. (TNIV)
Look at the logic. We're no longer condemned if we're in Christ. Why? All for whom Christ died have been transported to the realm of the life-giving Spirit. Why does this mean we're not condemned? Because those who walk in the Spirit are those for whom God fulfilled the righteous requirement of the law. Verse 4 is not prescriptive (i.e. walk in the Spirit and God won't condemn you) but descriptive (those who walk in the Spirit are those whom God has counted righteous).
The point of my argument is this, you can't have imputed righteousness without having been transported into the realm of the life-giving Spirit. This is the double grace (duplex gracia) of the work of Christ, i.e. justification and sanctification.
Robert Reymond quoting John Murray says that for the Christian there exists:
a cleavage, a breach, a translation as really and decisively true in the sphere of moral and religious relationship as in the ordinary experience of death. There is a once-for-all definitive and irreversible breach with the realm in which sin reigns in and unto death....In respect of every criterion by which moral and spiritual life is to be assessed, there is absolute differentiation. This means that there is a decisive and definitive breach with the power and service of sin in the case of everyone who has come under the control of the provisions of grace (A New Systematic Theology of the Christian Faith, p757)
Why bother with sanctification is a question asked by those who don't understand the nature of Christ's redeeming work. If you are in Christ, you have been justified for sure, but you have also had your old slave master, Sargeant Sin, killed. You have experienced a 'Definitive Sanctification' that is the alpha point for progressive sanctification. You now breath the air of the life-giving Spirit, and the fruit of this is a life of killing sin's deeds and fruit bearing.