Baptism in the Holy Spirit refers to the time in a Christian’s life when they receive the Holy Spirit into their lives.
The Holy Spirit was made available to the church on the day of Pentecost.
1When the day of Pentecost came, they were all together in one place. 2Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting. 3They saw what seemed to be tongues of fire that separated and came to rest on each of them. 4All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them.
Acts 2:1-4 (NIV)