Jesus had promised to pour out the Holy Spirit upon His disciples when He ascended into heaven. That promise is fulfilled at Pentecost. Here, they are fully prepared to take up the ministry for which Jesus prepared them during His earthly ministry.
At Pentecost, the Holy Ghost made a rare visible appearance as tongues of fire that burned over the heads of the disciples. By revealing Himself in this manner, just as He revealed Himself in the form of a dove at Jesus' baptism, He becomes recognizable with the Father and the Son as God. By sending the Holy Spirit, Christ revealed the reality of His resurrection from the dead and His ascension into heaven. And Christ sent the Holy Spirit to accomplish the work of revealing the life and work of Jesus Christ to all nations.
"For the disciples, this not only equipped them for their great office, but it also transformed their human weakness and despondency into divine strength and unconquerable courage. Their imperfect knowledge was changed into infallible, perfect enlightenment. Their simplicity and awkwardness were turned into heavenly wisdom." (CFW Walther)
The visible manifestation of the Holy Spirit set the disciples apart as those whom God had chosen to be His spokesmen on earth. God had called them to be Apostles and sent them out to all nations as teachers. While it is true that the apostles were given extraordinary gifts by the Holy Spirit on that first outpouring at Pentecost, the Holy Spirit comes and pours His gifts to each and every believer in Christ. He comes and makes His dwelling with the Father and the Son within us bringing us into the Kingdom of God.
As Luther's explanation to the Creed states, "I believe that I cannot by my own reason or strength believe in Jesus Christ, my Lord, or come to Him; but the Holy Ghost has called me by the Gospel, enlightened me with His gifts, sanctified and kept me in the true faith; even as He calls, gathers, enlightens, and sanctifies the whole Christian Church on earth, and keeps it with Jesus Christ in the one true faith."