Classical Pentecostal theology treats Spirit baptism as logically and normally chronologically distinct from conversion: a believer is first born again through faith in Christ, and then, often later and sometimes much later, receives a separate empowering baptism in the Holy Spirit, sought through prayer, often at a revival meeting or an altar service set aside for the purpose. This differs from most other Protestant theology, which locates the gift of the Spirit at conversion itself and does not expect or require a later, separate experience.
The practical effect has been to give Pentecostal worship a structure built around expectancy: services regularly include extended prayer specifically for people seeking Spirit baptism, testimony from those who have received it, and instruction in what the experience should look like. Believers who have not yet spoken in tongues are, in classical Pentecostal churches, understood to be seeking a real and attainable further gift rather than something already fully theirs.
The doctrine spread from its Pentecostal denominations into the Charismatic movement of the 1960s and 1970s, which carried Spirit baptism and the associated gifts into Catholic and mainline Protestant congregations, generally while relaxing the specific requirement that tongues alone serve as its evidence, a modification classical Pentecostal denominations have not generally accepted for themselves.