In Christianity, sanctification (or in its verb form, sanctify) literally means "to set apart for special use or purpose", that is, to make holy or sacred (compare Latin: sanctus). Therefore, sanctification refers to the state or process of being set apart, i.e. "made holy", as a vessel, full of the Holy Spirit. The term can be used to refer to objects which are set apart for special purposes, but the most common use within Christian theology is in reference to the change brought about by God in a believer, begun at the point of salvation and continuing throughout the life of the believer. When used in this sense, sanctification is known as growth in grace. Many forms of Christianity believe that this process will only be completed in Heaven, but some (particularly conservative Quaker and Methodist traditions, inclusive of the Holiness movement) believe that entire sanctification is possible in this life.

The Catholic Church upholds the doctrine of sanctification, teaching that:

Sanctifying grace is that grace which confers on our souls a new life, that is, sharing in the life of God. Our reconciliation with God, which the redemption of Christ has merited for us, finds its accomplishments in sanctifying grace. Through this most precious gift we participate in the divine life; we have the right to be called children of God. This grace is the source of all our supernatural merits and bestows upon us the right of eternal glory.

Saint Paul of the Cross stated that "Prayer, good reading, the frequent reception of the sacraments, with the proper dispositions, and particularly the flight of idleness—these are, believe me, the means of sanctifying yourself."

According to the Catholic Encyclopedia "sanctity" differs for God, individual, and corporate body. For God, it is God's unique absolute moral perfection. For the individual, it is a close union with God and the resulting moral perfection. It is essentially of God, by a divine gift. For a society, it is the ability to produce and secure holiness in its members, who display a real, not merely nominal, holiness. The Church's holiness is beyond human power, beyond natural power.

Orthodox Christianity teaches the doctrine of theosis, whereby humans take on divine properties, and in a particular sense, participate in the being of God. A key scripture supporting this is 2 Peter 1:4. In the 4th century, Athanasius of Alexandria taught that God became Man that man might become God. Essentially, man does not become divine, but in Christ can partake of divine nature. This Church's version of salvation restores the likeness of God in man. One such theme is release from mortality caused by desires of the world.

At the time of the justification of an individual, the Lutheran Churches teach that the process of sanctification commences, which is defined as "the Holy Spirit’s work which follows justification through faith and consists of renewing the believer and bringing forth in him works of renewal." In Lutheranism, sanctification has two components, including: "1.) The inner renewal of the Holy Spirit in the Christian, and 2.) the living out of that inner renewal in the Christian’s new life of good works." The Lutheran Confessions hold that it is "necessary to exhort people to Christian discipline and good works, and to remind them how necessary it is that they exercise themselves in good words as an evidence of their faith and their gratitude toward God". For Christians, "good works are necessary fruits of faith in the life of a Christian and that they proceed from a renewed heart that is thankful to God for His mercy and love". These good works done by Christians are rewarded by God. Those individuals who commit mortal sin "render themselves subject to divine wrath and eternal death unless, turned again, they are reconciled to God through faith." The Formula of Concord summarizes salvation in Lutheran Christianity:

First the Holy Spirit kindles faith in us in conversion through the hearing of the Gospel. Faith apprehends the grace of God in Christ whereby the person is justified. After the person is justified, the Holy Spirit next renews and sanctifies him, and from this renewal and sanctification the fruits of good works will follow.” (FC, Solid Declaration, Article III, Righteousness, 40,41 [Tappert])