Conversion to Christianity is the religious conversion of a previously non-Christian person to Christianity. Different Christian denominations may perform various different kinds of rituals or ceremonies of initiation into their community of believers. The most commonly accepted ritual of conversion in Christianity is through baptism, but this is not universally accepted among them all. A period of instruction and study almost always ensues before a person is formally converted into Christianity and becomes a church member, but the length of this period varies, sometimes as short as a few weeks and possibly less, and other times, up to as long as a year or possibly more.

Most mainline Christian denominations will accept conversion into other denominations as valid, so long as a baptism with water in the name of the Trinity took place, but some may accept a simple profession of faith in Jesus as Lord as being all that was needed for true conversion. Other Christians may not accept conversions performed in other denominations and certain communities may be discriminated against as heretical. This is true for many nontrinitarian sects, which many mainstream Christian denominations (Catholic, Orthodox, and Protestant) reject as having valid forms of conversion. Consequently, many nontrinitarian sects spiritually isolate themselves in that they may only consider their conversions valid and not those of mainstream Christianity.

Social scientists have shown great interest in the Christian conversion as a religious experience that believers describe as strengthening their faith and changing their lives. Christianization, defined as the "reformulation of social relations, cultural meanings, and personal experience in terms of (commonly accepted or supposed) Christian ideals", should be distinguished from conversion. Christianization is the broader cultural term, and typically has involved efforts to systematically convert an entire continent or culture from existing beliefs to Christianity.