Acts 1:1-3

The first account I made, Theophilus, was [a continuous report] about all the things that Jesus began to do and to teach [Luke 1:1-4] until the day when He ascended to heaven, after He had by the Holy Spirit given instruction to the apostles (special messengers) whom He had chosen. To these [men] He also showed Himself alive after His suffering [in Gethsemane and on the cross], by [a series of] many infallible proofs and unquestionable demonstrations, appearing to them over a period of forty days and talking to them about the things concerning the kingdom of God.