The former account I made, O Theophilus, of all that Jesus began both to do and teach, 2 until the day in which He was taken up, after He through the Holy Spirit had given commandments to the apostles whom He had chosen, 3 to whom He also presented Himself alive after His suffering by many infallible proofs, being seen by them during forty days and speaking of the things pertaining to the kingdom of God. — Acts 1:1-3.
In the first account, Luke wrote about all that Jesus began to do and teach until His ascension to heaven, after He had given commandments to the apostles He had chosen. After His suffering and death, Jesus presented Himself alive to these apostles with many convincing proofs over a forty-day period and spoke about matters concerning the kingdom of God.
He stresses that the facts of the Gospel message are reliable.