(Note: For a more technical treatment of this topic, see my article Justification Precedes Regeneration.)
In my article Romans 4:17-22 and the Demonstration of Abraham's Faith, I showed that both the Apostles Paul and James referred to Abraham's justification (i.e., imputed righteousness) in Genesis 15:6 as a singular event. I believe the argument I provided in that article is strong enough to dispel the idea that justification is an ongoing process that falls within the realm of sanctification.
However, there is an another way to show that justification is not part of the sanctification process. We can refute the idea that "the gospel is sanctification" if we can show from the Bible that God justifies ungodly people as ungodly people, i.e., as people who are not undergoing the sanctification process. If God justifies ungodly people as ungodly people, then this means that God accepts people apart from any good works (including "God-produced works" and "non-meritorious works"), and they could theoretically live their whole lives without producing a single good work and still be acceptable to God and welcomed into heaven upon death because of what Jesus did for them.
To the enemies of the Gospel, the thought of God justifying ungodly people as ungodly people is appalling. However, to people who are suffering under this predicament of sin and death that infects the whole human race, it is the best news ever, a true message of grace and mercy.
To start, let's look at Romans 4:5. In this verse we read, "But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness." This verse seems pretty straight forward. It teaches that God justifies the ungodly man as a man who does not work. In the context of Romans 1:18-3:31, "working" unquestionably includes "works of the moral law." So, Romans 4:5 essentially teaches that God justifies the ungodly man as an ungodly man. This means that justification is not part of the sanctification process, because sanctification involves the godly man as a godly man. Of course, there are times in the sanctification process where the godly man acts ungodly, but these acts of ungodliness are temporary set backs and do not characterize the sanctification process itself. Advances in sanctification always involve the godly man acting as a godly man.
The justification described in Romans 4:5 could more accurately be described as forensic justification. It is different from the moral/ethical justification described of Phinehas in Psalm 106:30 or Abraham in James 2:21. Paul Enns once described forensic justification as follows: "All, who have come to God through faith have been fully justified at the moment of faith. To justify is to declare righteous the one who has faith in Jesus Christ. It is a forensic (legal) act of God whereby He declares the believing sinner righteous on the basis of the blood of Christ." Opponents of forensic justification often pejoratively call it a legal fiction, but this is a misrepresentation because it does not tell the whole story and only tends to prejudice people's minds against the concept. A legal fiction refers to "accepting something as being a fact when in actuality it is otherwise." The truth is that there are legitimate legal fictions in the Bible. For example, the forgiveness of sins is a legal fiction, because the forgiver no longer holds a person guilty who in fact is guilty. The imputation of our sins to Christ (2 Corinthians 5:21) is a legal fiction, because God accepted Christ as being sinful when in actuality He was not. However, forensic justification is not a mere legal fiction. Believers in Jesus are imputed with righteousness. In other words, we receive an extrinsic righteousness, i.e., a righteousness that comes from outside of us.
Opponents of forensic justification have offered various objections to Romans 4 in order to interpret it in a manner more consistent with their belief that justification is part of the sanctification process. However, I have found none of their objections persuasive. For more details on their objections and my responses to their objections, please see the following articles:
Hebrews 11:8-9 and Imputed Righteousness
Romans 4:17-22 and the Demonstration of Abraham's Faith
More Objections Answered regarding Romans 4
Even More Objections Answered regarding Romans 4
In Romans 5:6-10 we read, 6 For when we were yet without strength, in due time Christ died for the ungodly. 7 For scarcely for a righteous man will one die: yet peradventure for a good man some would even dare to die. 8 But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us. 9 Much more then, being now justified by his blood, we shall be saved from wrath through him. 10 For if, when we were enemies, we were reconciled to God by the death of his Son, much more, being reconciled, we shall be saved by his life. From these verses we see that God loved us, justified us, and reconciled us not because of something He did in us to morally transform us. Rather, He did it while we were still weak and ungodly, still sinners, and His enemies. This shows what magnificent love, grace, and mercy God had for us at the cross. In the face of such hostility of sinners (Hebrews 12:3 NASB), He did all these things for us. Romans 5:6-10 is describing positional forensic justification, which refers to us becoming justified from God's point of view in time at the cross by the death of His Son in consideration of his choosing us in Christ before the foundation of the world (i.e., divine foreknowledge, as described in Romans 8:29, Ephesians 1:4, etc.). None of the Romans had believed in Jesus until many years after the cross. When they believed, in the manner described in Romans 4:5 above, they experienced forensic justification on an individual basis. In other words, the justification that was already accomplished for them at the cross was applied to them personally when the believed the good news of Jesus Christ. So, what we see is God accomplishing our justification in time at the cross (i.e., positional forensic justification) and then applying it to us experientially when we actually believe (i.e., experiential forensic justification). This creates a perfect harmony in God's plan. He justified us at the cross while we were yet viewed as ungodly and then applied it to our lives individually while we were yet ungodly.
In light of the above, it should be obvious how different the Apostle Paul's description of justification is compared to opponents of forensic justification. They insist that people are justified on the basis of moral transformation. In sharp contrast, Paul teaches that people are justified prior to any moral transformation as ungodly people who are yet sinners and enemies of God.