What is baptism in the Holy Spirit?
http://www.carm.org/questions/baptism_Spirit.htm

Baptism of the Holy Spirit1 is a term used to describe a movement of the Spirit upon and/or within a believer usually sometime after the person is saved. There is controversy surrounding this phenomenon as to whether it is legitimate or not. Some people believe that once a person is saved the Holy Spirit is in the person and there is no subsequent "baptism in the Holy Spirit." In other words, they maintain that this Baptism of the Spirit occurs at salvation. Others believe that it is possible for the Christian to experience an additional movement of the Holy Spirit sometime after salvation. Generally speaking, it is the charismatic movement that supports the Baptism of the Holy Spirit.
We need to first know that all Christians receive the Spirit upon their conversion and in this sense all Christians have been baptized in the Holy Spirit. This means that they are saved and that they have all they need at that time to be able to live godly and holy lives. 1 Cor. 12:13 says, "For by one Spirit we were all baptized into one body, whether Jews or Greeks, whether slaves or free, and we were all made to drink of one Spirit."
However, there are many Christians who claim to have had this "secondary" experience of the Spirit.2 They say that it has brought great blessing and comfort to them. Furthermore, they say that the results of the experience is a renewed dedication and appreciation for God, a stronger desire to read the Bible, a stronger desire to fellowship with Christians, and a deeper sense of worship of God. Millions of Christians who claim to have had this experience forces us to deal with the issue. Is it real or not? Let's look at the ????ure to find out.
The term "baptize with the Holy Spirit1" occurs several times in ????ure:

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Matt. 3:11, “As for me, I baptize you with water for repentance, but He who is coming after me is mightier than I, and I am not fit to remove His sandals; He will baptize you with the Holy Spirit and fire."
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Mark 1:8, "I baptized you with water; but He will baptize you with the Holy Spirit.”
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Luke 3:16, "John answered and said to them all, “As for me, I baptize you with water; but One is coming who is mightier than I, and I am not fit to untie the thong of His sandals; He will baptize you with the Holy Spirit and fire."
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John 1:33, "And I did not recognize Him, but He who sent me to baptize in water said to me, ‘He upon whom you see the Spirit descending and remaining upon Him, this is the one who baptizes in the Holy Spirit.’"
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Acts 1:5, "for John baptized with water, but you shall be baptized with the Holy Spirit not many days from now.”
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Acts 11:16, "And I remembered the word of the Lord, how He used to say, ‘John baptized with water, but you shall be baptized with the Holy Spirit.’"

We can clearly see that the phrase is used in the Bible. But, we do not find a clear teaching in the Bible of what the phrase means. Nevertheless, we can conclude that when a person is baptized in the Holy Spirit he has power bestowed upon him. This power is for the purpose of the preaching of the gospel (Acts 4:31), living a purer life, and having a deeper devotion to God. Also, it is frequently accompanied by speaking in tongues. Acts 2:4, "And they were all filled with the Holy Spirit and began to speak with other tongues, as the Spirit was giving them utterance." At this point, I would recommend the reader to examine Acts 1-2 to see the movement of the Holy Spirit upon the early church at Pentecost.
The issue now seems to be whether or not Baptism of/in/with the Holy Spirit is a subsequent event occurring after salvation. It would seem that this is the case. In John 20:22, Jesus commanded that the disciples receive the Holy Spirit, "And when He had said this, He breathed on them, and *said to them, “Receive the Holy Spirit." This means that they were saved since the Holy Spirit is not received by the unregenerate. Then, later in Acts 1:4-5 we read, "And gathering them together, He commanded them not to leave Jerusalem, but to wait for what the Father had promised, “Which,” He said, “you heard of from Me; 5 for John baptized with water, but you shall be baptized with the Holy Spirit not many days from now.”

Conclusion

The danger of this phenomena is the potential division of the body of Christ into two categories: those who are "regular" Christians and those who have been baptized in the Holy Spirit. This, of course, would be an incorrect way of looking at Christians, and this is why. If you were to step outside into a soft mist, it would take a long time to get completely wet. On the other hand, if you were to step into a torrential rain, you'd be drenched quickly.
Those who have not experienced the Baptism of the Holy Spirit (meaning a sudden and powerful experience) are not second class citizens by any means. They are the ones in the gentle mist who experience the Lord over a long period of time and get just as blessed as those who suddenly step into the torrent of the Spirit's presence. In fact, the Baptism of the Spirit can be a pitfall since so many people who have experienced it long for it again, almost to the point of putting the validity of their faith in the experience instead of the clear teaching of the word of God.
We must all be careful not to fall in our strengths as well as our weaknesses.

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1. Baptism of the Holy Spirit is also rendered as Baptism in the Holy Spirit and Baptism with the Holy Spirit
2. My own experience supports the idea that the baptism in the Holy Spirit can be a subsequent occurrence to salvation. Long after my becoming a Christian, I had an experience where the Holy Spirit moved upon me with great power. The result was an insatiable desire to read God's word, to hear praise music, and to speak of Christ. This had a profound and very long-lasting effect on my Christian life. Of course, experience is not what makes doctrinal truth. We find that in the word of God.