The Acts of the Apostles is a book that doesn’t quite fit with the other New Testament books. It’s not a Gospel, nor is it a doctrinal treatise like the Epistles. It’s just a bit of history sandwiched in between. That is not to say that it’s any less important than the rest of the Bible (Pentecost, anyone?); it’s just a bit different, and, I think, is probably rather neglected at times.

In the early days of the forming of the New Testament canon, the place of Acts was undetermined. The Gospels had been collected and bound in codex (book) form early in the second century AD. The Pauline Epistles comprised a second, separate collection. The book of Acts brought the two together.

The gospel collection was authoritative because it preserved the words of Jesus, than whom the church know no higher authority. The Pauline collection was authoritative because it preserved the teaching of one whose authority as the apostle of Jesus Christ to the Gentiles was acknowledged (except by those who refused to recognize his commission) as second only to the Lord’s. The bringing together of these two collections into something approximating the New Testament as we know it was facilitated by another document which linked the one to the other. This document was the Acts of the Apostles, which had been severed from its natural companion, the Gospel of Luke, when that gospel was incorporated in the fourfold collection. Acts had thereafter to play a part of its own, and an important part it proved to be. ‘A canon which comprised only the four Gospels and Pauline Epistles’, said Harnack, ‘would have at best an edifice of two wings without the central structure, and therefore incomplete and uninhabitable’ [A. Harnack, History of Dogma, E. T., II (London, 1896), p. 48, n.2.].

—F. F. Bruce, The Canon of Scripture (InterVarsity Press, 1988), 132–133.