Acts 1:4, 5, “…wait for the promise of the Father…for John baptized with water, but you will be baptized with (in) the Holy Spirit. Verse 4 tells us that this was a promise from God.
The Greek word for baptize is Strong’s #907 baptizo, and it means to make whelmed (fully wet). Spirit is Strong’s Greek word #4151, pneuma, and that refers to a current of air, breath, breeze, spirit, etc. Essentially, it represents an invisible force or power. So, the Holy Spirit is the power of God starting a new life in the believer which transforms the character into Christ-likeness. Romans 12:1, 2, “…present your bodies as a living sacrifice…be transformed by the renewal of your mind…” Galatians 5:22 – 25, “…the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control…if we live by the Spirit, by the Spirit let us also walk.”
This gift was given to the apostles and those gathered in the upper room on the day of Pentecost, fifty days after Christ's resurrection and after his ascension. Acts 2:2, “and suddenly there came a sound from heaven a sound like a mighty rushing wind, and it filled the entire house where they were staying.” At this point, the believers were baptized with the Holy Spirit and started a new life in the Lord.