Since the second century A.D., the book of Acts has commonly been called, "The Acts of the Apostles." In comprehending more completely and correctly the contents of the book, we may more accurately say that it is "The Acts of the Holy Spirit Through the Believers." We find that ten of the apostles' "ministries" and "acts" are never mentioned; whereas, several "acts" of non-apostolic believers are mentioned (Ac 8:5-8; 9:10-11, and 17).
The book of Acts is the greatest handbook of information on the workings of the Holy Spirit in the world today. It is the practical working out of the Great Commission (Mt 28:18-20, Mr 16:15-20, and Lu 24:46-49) and literally fulfills Mr 16:20, "They went forth and preached every where, the Lord working with them, and confirming the word with signs following." The Greek word for "working" in that verse is "SUNERGEO," and it literally means "to be a fellow-worker" (Strong's Concordance). This relationship between the Holy Spirit and the believers is clearly portrayed throughout the book of Acts.
Whereas "the former treatise" (that is, the Gospel of Luke) dealt with "all that Jesus began both to do and teach" (Ac 1:1), the book of Acts describes what Jesus continues to do and teach through the lives of the believers (His church). We might say, "These are the acts of the resurrected Christ through the believers."
As a result of the outpouring of the Holy Spirit, the witness of Christ and His teachings spread through Jerusalem, Judea, Samaria, and into the uttermost parts of the earth (Ac 1:8). Certainly one of the aims of this book was to show that the Jewish Messiah and His atonement were for all people, for all time.