Conversion to Christianity is the religious conversion of a previously non-Christian person that brings about changes in what sociologists refer to as the convert's "root reality" including their social behaviors, thinking and ethics. The sociology of religion indicates religious conversion was an important factor in the emergence of civilization and the making of the modern world. Conversion is the most studied aspect of religion by psychologists of religion, but there is still very little actual data available.

Christianity is growing rapidly in the global South and East, primarily through conversion. Different methods of conversion have been practiced historically. There is evidence of coercion by secular leaders in the Early and Late Middle Ages, though coercion as a method has never been approved or even supported by any majority of Christian theologians.^{[citation needed]}

Different Christian denominations may perform various different kinds of rituals or ceremonies of initiation into their community of believers. The primary ritual of conversion is baptism, while different denominations differ with regards to confirmation.

According to a 2001 study by religion professor David B. Barrett of Columbia University and historian George Thomas Kurian, approximately 2.7 million people were converted to Christianity that year from another religion, while approximately 3.8 million people overall were converting annually. In the first decades of the twenty-first century, Pentecostalism is the largest and fastest growing form of Christianity; this growth is primarily due to religious conversion.

James P. Hanigan writes that individual conversion is the foundational experience and the central message of Christianity, adding that Christian conversion begins with an experience of being "thrown off balance" through cognitive and psychological "disequilibrium", followed by an "awakening" of consciousness and a new awareness of God. Hanigan compares it to "death and rebirth, a turning away..., a putting off of the old..., a change of mind and heart". The person responds by acknowledging and confessing personal lostness and sinfulness, and then accepting a call to holiness thus restoring balance. This initial internal conversion is followed by practices that further the process of conversion, which according to Hanigan, will include ethical changes.

In examples of conversion from the New Testament, such as Peter's conversion and Paul's, Hanigan perceives this same common "death and rebirth" experience. He says these individuals did not respond out of a sense of guilt, but from awe, reverence, and holy fear of what they perceived as God's presence. Comparative studies of the early twenty-first century offer the insight that religious conversion provides a new locus of self-definition, moral authority and social identity through the acceptance of religious actions that seem more fitting and true to the recipient.

Religious conversion into Christianity sometimes came with physical incentives and rewards for new converts, such as the right of residence, access to land, or preferential legal status.

Anthropologist Robert Hefner adds that "Conversion assumes a variety of forms... because it is influenced by a larger interplay of identity, politics and morality". The message of Truth, a redemptive identity, and acceptance into a social organization whose purpose is the propagation of that message has proven to be a revolutionary force in its own right.