In Christian theology, Baptism of the Holy Spirit is a second baptism, "in fire," spoken of by Jesus in the Gospels. Specifically, it refers to the experience of Pentecost described in the Book of Acts. In most mainstream Christian churches, Pentecost is seen as a single act that spread out the Holy Spirit or paraclete onto all believers. Individuals thereafter might or might not have similar experiences, but the single event of Pentecost itself was sufficient for all time and to ensure that all future baptisms would convey the gift of grace.

However, a belief in a personal experience of revelation and renewal has been a feature of Protestant churches since the time of John Calvin, and a number of Protestant churches have adopted beliefs that might be generally called pentecostal or charismatic. For these churches, believers must experience a gift of the Holy Spirit, either after or even before regular baptism with water.

In contemporary theology, there is a divergence between the two main strains of pentecostal believers, with some organized as Pentecostal and others as Charismatic churches. Both believe that the baptism of the Holy Spirit is spoken of by Jesus in Luke 11:13 and also Acts 1:5 and that it was the outpouring of the Holy Spirit prophesied in the Old Testament books of Ezekiel 36:27 and Joel 2:28-29. Both of these strains of Protestantism diverge from other churches in the essential nature of grace and what grace is granted without an individualized experience of the Holy Spirit.

Pentecostal/Charismatic View

Charismatics and Pentecostals both point to Ephesians 5:18, where the Apostle Paul urges his audience to "be filled with the Spirit" using an imperative mood verb. Pentecostalists see this as a gift and an experience different from that of salvation, yet which follows salvation. Whereas other churches have seen being filled with the Holy Spirit to require piety and grace, Pentecostals and Charismatics have seen it as a requirement that all who are saved must have a pentecostal experience.

Charismatics and Pentecostals differ from one another in the evidence they require for proof of baptism in the Holy Spirit. Charismatics will look for the "fruit of the spirit" spoken of in Galatians 5:22_25, and the Pentecostals will look for glossalalia, prophecy, and other "gifts of the spirit" described in Acts 2:1_4.

This was, according to Pentecostals, the normal experience of all in the early Christian Church. With it comes the endowment of power for life and service, the bestowment of the gifts and their uses in the work of the ministry (Luke 24:29; Acts 1:4-8; 1 Corinthians 12:1_31).

Not all evangelical Pentecostal churches would accept that all Christians receive the Holy Spirit at the time of their conversion or baptism, for instance the Apostolic Assemblies of Christ, but in the more traditional evangelical point of view, and in non-evangelical churches, the Baptism in the Holy Spirit is equated with this reception. Others, even outside the Pentecostal church, consider the Baptism of the Holy Spirit as a separate experience. Even among those who accept this, opinion is divided as to whether all those who receive the Baptism also receive the gift of tongues.

Both Pentecostal and Charismatic churches regard the bapitsm of the Holy Spirit to be requisite for the apostolic and evangelical mission that they believe is the duty of all Christians.

Other relevant Bible passages include Acts 8:14-17, Acts 2:1-13. See also Criticisms of Pentecostal and Charismatic belief.