"When the day of Pentecost had come, they were all together in one place. And suddenly a sound came from heaven like the rush of a mighty wind, and it filled all the house where they were sitting. And there appeared to them tongues as of fire, distributed and resting on each one of them. And they were all filled with the Holy Spirit and began to speak in other tongues, as the Spirit gave them utterance."
-Acts 2:1-4 (RSV)
We have now done a high level overview of how Pentecostalism really came to be a distinct movement in Christianity. We know that it grew out of holiness movement, and that the two most significant figures in the emergence of Pentecostalism were Charles Fox Parham and William Joseph Seymour. It was Parham's doctrine of tongues-attested baptism in the Holy Spirit that led to the Azusa Street Revival and the flourishing of the Pentecostal Movement. But now we need to take a closer look at this doctrine. What is "Holy Ghost Baptism"? What does it do? Why are tongues considered the initial evidence of it? And, for that matter, what are tongues?
For our purposes, we will focus in on how the Assemblies of God approaches these questions. The Assemblies of God stands as the largest of any Pentecostal denomination, by a large margin, and has made perhaps the most rigorous attempts at doctrinal definition than any other denomination. It must be noted that one key difference that you will find in Wesleyan-Pentecostalism is the emphasis on Baptism in the Holy Spirit as the moment of entire sanctification. The believer, in this moment, definitively enters a new state by faith. The Assemblies of God, on the other hand, adheres to a progressive sanctification model which takes a more (broadly) Calvinistic approach on this event, a la the Higher Life Movement, with an emphasis on empowerment that allows the believer to more fully realize the process of sanctification. Without getting too bogged down in the details, as they are not relevant to the topic at hand, the Wesleyan-Pentecostal view focuses on absolute purity, while the non-Wesleyan-Pentecostal view focuses on empowerment.
Anyway, back to the topic.
What is Holy Ghost Baptism?
To get a full sense of what is meant by “Baptism in the Holy Spirit”, we must first make a distinction between two similar, but different terms. The General Presbytery of the Assemblies of God makes clear that the Baptism in the Holy Spirit is not the baptism by which one enters into the Body of Christ. Paul tells us in 1 Corinthians 12:13 that we are all baptized by one Spirit, which the General Presbytery notes means that the Holy Spirit is most certainly the instrument of our baptism. The baptism Paul is speaking about here is the very same he speaks about again in Ephesians—the baptism which results in the one Body of Christ. The General Presbytery refers to this baptism Paul is speaking about as the Baptism of the Holy Spirit, and thus we see the distinction. The Baptism of the Holy Spirit is the means by which the believer enters into the Church, while Baptism in the Holy Spirit is a subsequent event which occurs in the life of one already baptized into the Body.
The subsequence of the Baptism in the Holy Spirit is of primary importance to Pentecostal theology. The General Presbytery points to the event at Pentecost in Acts 2 in order to emphasize this subsequence. The disciples had already been converted to the faith—they had already received the Baptism of the Holy Spirit, as Jesus tells them that their names are written in Heaven, and later breathes on them with the words “Receive the Holy Spirit”. Thus, what occurred at Pentecost was by no means the regenerative grace of the “one baptism”, but was rather something subsequent which took place in and to the already-regenerate believers. The General Presbytery points to the conversion of Paul as a primary example of this subsequence, as Paul had already been converted to the faith through his encounter with Jesus on the road to Damascus and it was three days later that Paul was filled with the Holy Spirit (received the Baptism in the Holy Spirit) through the laying on of hands by Ananias.
There is another key element to the Pentecostal understanding of Baptism in the Holy Spirit beyond the subsequence of the event, and that is that the experience is always accompanied by the speaking in tongues.
The late Dr. Stanley M. Horton, a Pentecostal theologian from the Assemblies of God, describes this evidence of Baptism in the Holy Spirit with a powerful anecdote. He tells of one of his father’s friends who went to a Pentecostal tent meeting in Vancouver. Wanting to receive the baptism of the Holy Spirit through the experience of a great violent wind, as it is described in the book of Acts. He was told to go pray for the gift, and was physically lifted up by a wind and came to lie on the far end of the tent. Immediately wanting to go and tell others about his baptism in the Holy Spirit, he found he could not get up—he felt as if he was nailed to the floor. It was then when he began speaking to speak in tongues. “He had no doubts after that about tongues being the initial outward evidence of the baptism in the Holy Spirit.”
“And they were all filled with the Spirit [at Pentecost]. What was the immediate result and the outward evidence of that filling? They began to speak with other tongues.” This quote comes from one of Aimee Semple McPherson’s sermons. McPherson stands as one of the most prominent figures in Pentecostalism, founding the International Church of the Foursquare Gospel and known worldwide in the early 20th Century for her “spirited” sermons (pun intended) and her miraculous faith healings. Later in this same sermon, she points to Paul’s encounter with the people at Ephesus. McPherson recalls that when Paul laid his hands on these Ephesians, when the Holy Spirit came upon them, they spoke in tongues just as the disciples had at Pentecost. Further Scriptural evidence of this accompaniment is provided by Horton, who points to Paul being filled with the Spirit in Acts 9, and Paul’s own statement that he has the gift of tongues.
What must be further clarified is that the gift of tongues is, as it was at Pentecost, the gift of some real language. As Horton tells us, “Certainly, babbling could not be considered a gift or work of the Holy Spirit.” Horton holds firmly that the gift of tongues is always the gift of a human language. However, this is not explicitly stated by the General Presbytery, who seem to leave open the possibility, rejected by Horton, for the bestowing of angelic languages. It seems that if the believers were to receive angelic languages, the words they speak would be no less “real”—they would hardly be mere babbling—and so Pentecostalism, dogmatically speaking, leaves the question open-ended as to whether or not believers can receive an angelic language rather than a human language (while certain Pentecostal theologians prefer to take a more hard line position on the subject).
What does Holy Ghost Baptism do?
Now with a firm understanding of what the Baptism in the Holy Spirit is, we ought to take a look at what it does to the believer. What kind of transformation takes place and, if this is to be understood as an empowering by the Holy Spirit (as already mentioned), what does “empowerment” mean?
The General Presbytery points to four “Divinely-intended results of Spirit Baptism”: the gift of tongues, openness to spiritual manifestations, righteous living, and power for witnessing.
That tongues is the initial physical evidence of Spirit Baptism has already been made clear, but the General Presbytery makes a distinction between tongues as evidence of baptism and tongues as the gift spoken about by Paul. Thus when we are looking at these “Divinely-intended results”, we are speaking about the gift of tongues, not the initial empirical evidence of Spirit Baptism. The Presbytery notes three purposes of this gift. There is the devotional aspect, where the believer is empowered to more fervent and efficacious prayer, and in this sense the purpose is called a “prayer language”. There is the purpose for the edification of the Church, and this is found in the speaking of tongues when accompanied by the interpretation of tongues, spoken of by Paul in 1 Corinthians 12. The gift of tongues also stands as a sign for unbelievers, as we see at Pentecost in Acts 2, where the unbelievers were amazed by the disciples.
Following directly from Paul, the General Presbytery describes the “openness to spiritual manifestations” as the opening up of the one baptized to the full range of the spiritual gifts. This baptism also results in an intensified dedication to the faith—“an added impetus to pursue a life pleasing to God.” It further empowers the believer with abilities related to evangelization, beyond the third purpose of the gift of tongues. The believer is endowed with power for the proclamation of the Gospel in a more efficacious manner, and in this we find several distinct powers which are given: the gift of preaching, the power to perform exorcisms, the power to raise from the dead, and the power of healing.
And so we have come to the end of my explanation of Pentecostal theology surrounding Holy Ghost Baptism. This is not intended to be exhaustive, indeed dozens of theologians have written entire tomes on this topic. But it should provide you with a simple bread-and-butter understanding of the single most fundamental doctrine of Pentecostalism. If there are any questions of clarification, or if you are simply lost altogether, please post a comment below and I can elaborate on anything you'd like!
Thus concludes Part II to our Pentecostalism Series. In Part III, I will be doing some reflection on the doctrine we have just discussed, with the help of perhaps the most prominent figure in Catholic pneumatology in the 20th Century, Yves Congar.
 The General Presbytery of the Assemblies of God. "Baptism in the Holy Spirit." August 11, 2010. 1
 Luke 10:20
 John 20:22
 Ephesians 4:5; At this point it is worth mentioning that the regenerative power of baptism is not denied by Pentecostal theology. Rather the moment at which this occurs is shifted from ritual water baptism to the moment of conversion, which is the Baptism of the Holy Spirit. It is through conversion that the believer enters into the Body, when the Holy Spirit indwells in the believer
 Horton, Stanley M., Larry Hart, H. Ray Dunning, Ralph Del Colle, and Walter C. Kaiser, Jr. Perspectives on Spirit Baptism: Five Views. Kindle ed. Compiled by Chad Brand. Nashville, TN: Broadman & Holman Publishers, 2004. 56.; The General Presbytery. 5
 Acts 9:1-19
 The General Presbytery. 5; Acts 9:17-19
 Acts 2:1-5
 Horton, 47
 McPherson, Aimee Semple. This Is That: Personal Experiences, Sermons and Writings. Los Angeles: Echo Park Evangelistic Association, 1923. 418
 Acts 19:1-7; McPherson, 423
 1 Cor. 14:18
 Horton, 71
 Ibid, 72
 Horton’s rejection seems suspect in itself, as he seems to reject angelic languages on the basis that the use of “tongues of angels” in Scripture might be a figure of speech, and that there are “plenty [of human languages] to choose from.”
 General Presbytery, 10
 General Presbytery, 10; Horton, 76
 The distinction comes as a result of Paul’s statement in 1 Cor. 12:30: “Not all speak in tongues, do they?” While speaking in tongues is always the evidence of baptism in the Holy Spirit, the continued gift of tongues is only bestowed upon some.
 General Presbytery, 10-11
 Ibid, 11