In Christian theology, justification is God's righteous act of removing the condemnation, guilt, and penalty of sin, by grace, while, at the same time, declaring the unrighteous to be righteous, through faith in Christ's atoning sacrifice.
The means of justification is an area of significant difference amongst the diverse theories of atonement defended within Roman Catholic, Eastern Orthodox and Protestant theologies. Justification is often seen as being the theological fault line that divided Roman Catholicism from the Lutheran and Reformed traditions of Protestantism during the Reformation.
Broadly speaking, Catholic, Methodist and Orthodox Christians distinguish between initial justification, which in their view ordinarily occurs at baptism (as with the Catholicism and Eastern Orthodoxy) or the New Birth (as with Methodism), and final salvation, accomplished after a lifetime of striving to do God's will (sanctification).
In Catholic doctrine, righteousness is "infused", i.e., God "pours" grace into one's soul or, "fills" one with his grace more and more over time; faith as is shown through charity and good works (fides caritate formata) justifies sinners.
In Lutheran and Reformed doctrine, righteousness is imputed (λογίζομαι, "logizomai") to the inherently ungodly, by grace, through faith in the cross of Christ. Reformed Christianity teaches the concept of fiduciary faith, that is, that "faith alone suffices for justification, and that consequently the observance of the moral law is not necessary either as a prerequisite for obtaining justification or as a means for preserving it." Therefore, a righteousness from God is viewed as being credited to the sinner's account through faith alone, apart from works, being based solely on the blood of Christ.
In Methodist doctrine, imputed righteousness is received during justification, which happens in the New Birth; imparted righteousness is communicated through sanctification. Partaking in the means of grace (works of piety and works of mercy) are integral to sanctification, as is the keeping of the moral law contained in the Ten Commandments. The desired state is Christian perfection, also known as entire sanctification (the second work of grace), described as being "habitually filled with the love of God and neighbor" and as "having the mind of Christ and walking as he walked".
In Roman Catholic, Lutheran, and Reformed doctrine, as well as in Methodism and Eastern Orthodoxy, anyone who has been justified will produce good works as a product of faith, just as a good tree produces good fruit.
For Lutherans, justification can be lost with the loss of faith; for Catholics, justification can be lost by mortal sin. For Methodists (inclusive of the holiness movement), salvation can be lost with the loss of faith or through sinning (cf. conditional security). The Reformed tradition generally holds that justification can never truly be lost: for those who have been justified by grace, will certainly persevere through faith, until the return of Christ himself.