Sanctification is the process by which a person is made holy. In Christian theology it is distinguished from justification, which is the declaring or making of a person righteous before God, and the classical Protestant statement of the difference is that justification is an act, complete, not admitting of degrees, while sanctification is a work, progressive and unfinished in this life. Catholic theology joins the two more closely, holding that justification itself involves an inward renewal and not only a change of standing, and much of the sixteenth-century argument turns on whether that is one doctrine or two.
The underlying scriptural idea is holiness as separation and belonging: Israel is holy because set apart to God (Leviticus 20:26), and the New Testament calls believers hagioi, holy ones or saints, as a description of their standing before it is a description of their attainment. Paul states sanctification both as accomplished, "you were washed, you were sanctified" (1 Corinthians 6:11), and as commanded, "this is the will of God, your sanctification" (1 Thessalonians 4:3). Every tradition has to hold those together, and they resolve the tension in different directions.
The principal division is whether entire sanctification, a real freedom from voluntary sin, is attainable in this life. John Wesley taught Christian perfection as a genuine possibility, understood as perfection in love rather than in knowledge or judgement; the nineteenth-century holiness movement made it central, and Pentecostalism emerged from that movement and reframed the second work as baptism in the Spirit with its own evidence. Reformed theology denies attainable perfection, holding that sin remains until death and reading the relevant texts as describing a direction rather than a reachable state. Lutheran theology tends to keep justification so firmly at the centre that sanctification appears as its fruit rather than as a separate programme, on the ground that making it a programme reintroduces works. Eastern Orthodoxy frames the whole matter differently again, as theosis, deification, participation in the uncreated energies, a process with no terminus, and generally regards the Western division between justification and sanctification as a scholastic distinction its own tradition has not needed.
The means are broadly agreed even where the theory is not: scripture, prayer, the sacraments or ordinances, the discipline of the community, and suffering. What differs is how much is expected, and the pastoral consequences are real, assurance and the lack of it, scrupulosity, and the recurring difficulty of how to speak honestly about continuing sin to people who have been told that they are holy.