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All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them. Acts 2:4

This verse and event in Acts is a fulfillment of the event Jesus spoke about in the Gospels, see March 16:17 as one example.  It is clear that God used the filling of the 120 disciples and the thousands who heard the Gospel preached for the first time that day as His way to supercharge the growth of the church.  These same disciples who were filled that day were the very same disciples who hid like a dog who just dragged toilet paper all over the house and was caught in the act by it’s owner, just fifty days prior.  By themselves they could not stand up and preach the Good News, but once they were filled with the Holy Spirit they were ready to proclaim the Gospel to all who were near and those who were far off.  This change is even more evident in the way the disciples died.  Almost all of them were martyred for proclaiming the name of Jesus to those who did not want to hear it.

My conclusion is that the baptism in the Holy Spirit is absolutely necessary to be a forceful witness for Christ in our modern age, just as it was in the first century after Christ.  The Lord used this event as a sign to the unbelievers in Jerusalem then and He continues to use this supernatural occurrence as a sign to unbelievers today.  Being filled with the Holy Spirit does not save you and is not required for salvation but it is absolutely necessary when it comes to witnessing and living a victorious Christian life!

The applications of the Baptism in the Holy Spirit is the same today as it was on that first Pentecost.  The infilling of the Holy Spirit gives a believer power to proclaim Christ, witness to unbelievers, and a faith that can move mountains.