In the first book, O Theophilus, I have dealt with all that Jesus began to do and teach, until the day when he was taken up, after he had given commands through the Holy Spirit to the apostles whom he had chosen (Acts 1:1–2 ESV).
“Luke’s first two verses are, therefore, extremely significant. It is no exaggeration to say that they set Christianity apart from all other religions. These regard their founder as having completed his ministry during his lifetime; Luke says Jesus only began his.
True, he finished the work of atonement, yet that end was also a beginning. For after his resurrection, ascension and gift of the Spirit he continued his work, first and foremost through the unique foundation ministry of his chosen apostles and subsequently through the post-apostolic church of every period and place.
This, then, is the kind of Jesus Christ we believe in: he is both the historical Jesus who lived and the contemporary Jesus who lives. The Jesus of history began his ministry on earth; the Christ of glory has been active through his Spirit ever since, according to his promise to be with his people ‘always, to the very end of the age’ (Matt 28:20).”
~John R. W. Stott, The Message of Acts: The Spirit, the Church and the World (Downers Grove: InterVarsity, 1990) p. 34.