Luke referred to the Gospel he had written to Theophilus.

1 Dear Theophilus,

In my first book that I wrote for you, I wrote about many of the things that Jesus did and taught 2 until the day on which he was taken {God took him} up to heaven. Before he went to heaven, saying what the Holy Spirit told him, he told the apostles whom he had chosen the things that he wanted them to know. 3 After he had suffered and died on the cross, he became alive again. As he appeared to them often during the next 40 days, the apostles saw him many times. He proved to them in many ways that he was alive again. He talked with them about how God would rule [MET] the lives of people who accepted him as their king.