Conversion is the adoption of a religious identity, whether by moving from one tradition to another, from none to one, or from nominal to committed practice within a tradition already held.
Its academic study begins with William James's Varieties of Religious Experience (1902), which distinguished the sudden, crisis-driven conversion of the "twice-born" from the gradual, undramatic development of the "once-born", and observed that the first has been far more studied without being the more common. The Pauline model, a sudden reversal on the Damascus road, has dominated Western conceptions of what conversion looks like, and Krister Stendahl argued influentially that even Paul's own account is better described as a prophetic call within Judaism than as a conversion away from it, which illustrates how strongly the paradigm shapes the reading of its own founding case.
Later scholarship has moved from psychological accounts of the individual toward social ones. Studies of new religious movements, notably Lofland and Stark's work from 1965, found that conversion typically follows pre-existing social ties, people join religions their friends and relatives already belong to, and that belief frequently follows affiliation rather than preceding it, which reverses the intuitive order. The "brainwashing" model, applied to new movements in the 1970s and used both in litigation and to justify coercive deprogramming, has been rejected by the great majority of scholars of religion and by most courts, and the deprogramming industry it supported was largely dismantled. That rejection does not entail that no group uses coercive or manipulative practices, and scholars are careful to distinguish the general theory, which failed, from the assessment of particular groups, which must be done case by case.
Traditions differ sharply in whether they seek converts. Christianity and Islam are the major missionary religions and between them account for most of the world's conversion. Judaism accepts converts but does not seek them and traditionally discourages an applicant initially. Hinduism historically had no conversion mechanism, and whether one can become a Hindu is genuinely contested within it. Zoroastrianism and the Druze do not accept converts at all, and the Parsi community's position on this is bound up with its demographic decline.
Conversion is politically consequential in a way that makes it a persistent flashpoint. Several Indian states have anti-conversion laws restricting conversion by force, fraud or inducement, which critics argue are used in practice to harass minorities and those who work with them. Several Muslim-majority states criminalise conversion from Islam. And the right to change one's religion, protected under Article 18 of the Universal Declaration of Human Rights, is among the most contested provisions of international human rights law precisely because it is the point at which individual freedom and communal continuity collide most directly.