The Holy Spirit did not make His first appearance at Pentecost. Students of the Bible will find Him mentioned as early as the creation account (Genesis 1:2). He is also shown to be doing the Father's work throughout the Old Testament and Gospels. However, the Spirit arrived in the upper room with a fresh mission from the Father.
In the Old Testament, the Holy Spirit empowered individuals for specific tasks. For example, Bezalel's God-given wisdom and craftsmanship enabled him to become the Tabernacle's chief architect (Exodus 31:1-5). We also read that the Spirit settled upon leaders needing help in administration (Numbers 11:16-17); warriors facing formidable tasks (Judges 6:34; 1 Samuel 16:13); and men called to proclaim God's word (Isaiah 61:1; Ezekiel 2:1-4). When the Lord wanted a job done, He chose someone to do it. Then the Holy Spirit equipped him or her for the task. He gave power only to certain people and didn't necessarily remain with them long.
Indicating that the Spirit's involvement with believers would be different than before, Jesus told the disciples, "He abides with you and will be in you" (John 14:17). God's Holy Spirit had come alongside the disciples during their time with Christ. But following the completion of Jesus' work on earth, He would dwell within them.
Since Pentecost, every believer has received the Holy Spirit. If Jesus is the vine and we are the branches, then the Spirit is the sap equipping us for the Christian life. Furthermore, He no longer comes and goes but rather remains permanently. He seals us in Christ—proof of the promise that we are forever in God's presence (Ephesians 1:13-14).
Taken from "The Holy Spirit—His Presence" by In Touch Ministries (used by permission).