This verse provides the underlying reason for the visible manifestations of power shown in Acts 2:1-6. The resurrected Christ tells the apostles, “But you shall receive power when the Holy Spirit has come upon you; and you shall be witnesses to Me in Jerusalem, and in all Judea and Samaria, and to the end of the earth.”
As in II Timothy 1:6-7, God's Spirit is linked with power—the effective capacity for God to work through a person. But this also shows what God intends when He gives the power of His Spirit: That person is to be a witness of Jesus Christ and ultimately of our Spiritual Father. This capability is not just for the apostles. We may not receive this power in the same dramatic way, and we may not be used in an apostolic role, but everyone who has received God's Spirit has the capacity to be a witness of God.
The Spirit of God, which is the same Spirit motivating Jesus Christ, imparts spiritual knowledge, understanding, and wisdom. It impels us toward holiness. It is a Spirit of power, love, and sound-mindedness. It is the essence of God's mind and enables the outworking of His will. God gives a measure of His own remarkable Spirit to incline His children to think the same way as He thinks and to live as He lives.
The more that we yield to, and make use of, God's Spirit, the more He gives. As we seek God's direction and instruction, and are careful not to quench or grieve the Spirit of our holy God (Ephesians 4:30), His character image takes shape in us. And as we grow in His image, we become witnesses of Him—our lives become testimonies of the goodness of God, the mind of God, the love of God, the holiness of God, the stability of God, and so much more. God gives us the essence of His mind so that we can reflect His glory to the world, through becoming just like Him.
David C. Grabbe
What Is the Holy Spirit?