Two question every Christian asks are, “what is God’s will for my life?” and “How can I follow God’s will for my life?” The early part of Acts addresses these very questions through the speeches Luke puts into writing.
When we omit the asides, Peter’s brief speech is not about Judas’s death but about the need to replace him. One of the major themes that resurfaces throughout Acts is enunciated in the very first words Peter utters in v. 16: “The scripture had to be fulfilled.…” The idea of divine necessity is a common one in ancient historiography, especially in Hellenistic historiography, but Luke’s interest is in a more comprehensive and detailed idea than just the Greco-Roman idea of fate. His interest also goes beyond the notion of divine providence, though that, too, is involved at various points in Luke-Acts. At this juncture what Peter is referring to is the plan of God set forth in the Holy Scriptures, which must be fulfilled. Luke is not talking about some hidden, inscrutable design of a deity, but rather about the revealed will of God. J. T. Squires helpfully sums things up as follows:
The theme of the plan of God plays an important role in Luke-Acts. The author refers to God’s guidance of events from the very beginning of the Gospel through to the end of Acts. At key points in the narrative, such divine control is emphasized; however, Luke intends throughout to convey the message that God’s guidance is comprehensive in scope and consistent in nature, underlying all the reported events.… Two particular events are emphasized as occurring within the plan of God: the crucifixion and the mission to the Gentiles. Both events may be construed as involving a change of course in that original divine plan: thus Luke carefully presents each event as integral to God’s purposes.… This theme functions apologetically to defend Christian beliefs and attack other views.… [The latter is] displayed most clearly in the speeches in which Stephen and Paul attack idolatry by using themes of divine providence which were commonly part of rhetorical techniques to attack idolatry in the Hellenistic world.
For our purposes what is crucial to note is that Peter is using Scripture apologetically to explain the surprising outcome of Judas’s life and the divine justification for his replacement. Of course the betrayal of the Messiah by one of his followers, leading to his death, required such an explanation, since this was no part of early Jewish messianic expectation. Judas is said not merely to have been the one who guided the arresters of Jesus (v. 16b), but also one “numbered among us … allotted his [lot] in this ministry.” Judas was not only not an outsider, he was one of the chosen Twelve who had shared in the ministry of Jesus and his followers.
Ben Witherington III, The Acts of the Apostles : A Socio-Rhetorical Commentary (Grand Rapids, MI: Wm. B. Eerdmans Publishing Co., 1998), 121-22.
God guides and orchestrates events, but often we do not have they right perspective to see just how He is at work.