Roman Catholic theology confuses justification with sanctification. On the other hand, many Evangelicals confuse justification with salvation as if the two were equivalent terms, resulting in an "incipient antinomianism" or lawlessness. In this video, R.C. Sproul explains that salvation is the point at which we repent, i.e. conversion; salvation is a process of sanctification (however imperfect that sanctification may be); and salvation is completed at the point of our death (often called "glorification"). Justification is a legal or forensic declaration of not guilty and the merits of Christ's perfect life are credited to us. Christ suffers in our place on the cross and bears all the penalty for our sins in the past, in the present, and in the future. This substitutionary atonement satisfies God's law, the just penalty of the law, and removes our guilt. The basis for our justification is not sanctification.
The sole basis for our justification is Christ and his holy life and his atoning death. However, our salvation includes all three aspects I mentioned earlier. That is, salvation is our justification, sanctification and our glorification. R.C. also mentions the ordo salutis. Roughly speaking, the ordo salutis is the logical and/or temporal order in which our salvation is unfolded. According to Reformed theology, this would be election, predestination, regeneration, conversion/repentance, justification, sanctification, and glorification.
One of the major issues leading to the division between Roman Catholicism and the Protestant Reformation is the distinction between justification and sanctification. Justification and sanctification must always be distinguished from each other. Justification is our legal declaration of "not guilty" based on the substitutionary atonement of Christ on the cross. Christ died for our sins to justify us before God in the final judgment. It is outside of us and it is an objective fact based on what Christ completed for us by meriting eternal life for us in his perfect obedience on earth and by dying in our place for our sins on the cross. Roman Catholicism committed the error of confusing sanctification, which is an inherent process inside our souls where we are made more and more like Christ as we live a Christian life, with justification.
The Roman Catholics can never be sure they are saved because they base their salvation on obedience rather than on justification by faith alone. For Roman Catholics justification is inherent in the soul and the sinner is made actually righteous through a subjective infusion of righteousness into the heart or mind. This flies in the face of Romans 7 where Paul clearly says that we remain sinners even after we have been legally declared righteous and begin the process of living a sanctified life. This is why low church Anglicanism has absolutely nothing in common with high church Anglicanism. High church Anglicanism commits the same theological error which the Roman Catholics have committed. It has confused justification with sanctification and ignores the fact that justification is a legal or forensic declaration and is absolutely not the process of being inherently and progressively sanctified in our walk with Christ.
Addendum: Salvation is all of God and even our sanctification is a monergistic work of God. We do not "cooperate" with God's grace even in sanctification and it is only God who keeps us from falling away into apostasy. God promises to keep the elect in spite of our capricious and sinful wills. While definitive sanctification is imputed along with our justification, definitive sanctification is not sinless perfectionism (Wesleyan entire sanctification). Definitive sanctification breaks the power of sin so that we can make meager baby steps in progressive sanctification but even our progress is nothing compared to God's holiness. It would be like trying to walk across the universe starting from earth. In other words, it is impossible to trust our progress in sanctification as any basis whatsoever for assurance of salvation. It is only the cross of Jesus Christ that saves us and that salvation was objectively accomplished on the cross 2,000 years ago. Charlie 03/02/2012
[Addendum: I don't erase older posts. I have been reading Dr. Gordon H. Clark's books and my views have changed. Dr. Clark says that regeneration and the perseverance of the saints are monergistic. But sanctification is synergistic because it is God who works in us to cause us to both to will and to do according to His good pleasure. Philippians 2:11-13. Also, the Westminster Confession of Faith does say that obedience gives us an assurance of our salvation. Charlie 3/22/2016]