“The former treatise have I made, O Theophilus, of all that Jesus began both to do and teach,”
King James Version (KJV)
1:1 The former treatise - In that important season which reached from the resurrection of Christ to his ascension, the former treatise ends, and this begins: this describing the Acts of the Holy Ghost, (by the apostles,) as that does the acts of Jesus Christ. Of all things - In a summary manner: which Jesus began to do - until the day - That is, of all things which Jesus did from the beginning till that day.
Ac 1:1 Waiting for the Holy Spirit SUMMARY OF ACTS 1: The Preface. The Command to Witness. The Ascension. The Charge of the Angels. The Week of Prayer. The Fate of Judas. The Choice of an Apostle. The former treatise. Luke's Gospel. Luke, whose history of Acts is really a continuation of the history of his Gospel, very naturally refers to the former. Theophilus. See PNT "Lu 1:3".