"But you will receive power when the Holy Spirit has come upon you, and you will be my witnesses in Jerusalem and in all Judea and Samaria, and to the end of the earth."

Jesus in Acts 1:8

During the season of Pentecost we celebrate the gift of the Holy Spirit which was poured out upon the church on that first Pentecost and continues to be poured out through God's Word and the water of Holy Baptism.  Martin Luther describes the power of the Holy Spirit in his explanation of the third article of the Apostles' Creed.

I believe that I cannot by my own reason or strength believe in Jesus Christ, my Lord, or come to Him; but the Holy Spirit has called me by the Gospel, enlightened me with His gifts, sanctified and kept me in the true faith.

In the same way He calls, gathers, enlightens, and sanctifies the whole Christian church on earth, and keeps it with Jesus Christ in the one true faith.

In this Christian church He daily and richly forgives all my sins and the sins of all believers.

On the Last Day He will raise me and all the dead, and give eternal life to me and all believers in Christ.