In the days leading up to Pentecost, Luke describes the disciples of Jesus as being of “one mind.” This nouns (ὁμοθυμαδόν) is repeated in 2:46 (translated simply as “together” in the ESV) and 4:24 to describe the worship of the apostolic community. As Keener notices, the word forms an inclusio, framing the events of Pentecost with the idea of the unity of this early community of believers (Acts, 1:751). In fact, the unity of the early community is an important theme in Acts.
The word means “one passion” (ὁμός, “common,” and θυμός, “passion, anger”), and can be used for any group that has a single interest, whether for good or bad. For example, in Acts 8:6 it describes the crowd paying close attention to Philip the Evangelist). In Acts 15:25 the word is used for the unanimous decision of the Jerusalem council.
But the word is not always positive since the crowd in Acts 7:57 were also “of one mind” when rushed out to stone Stephen, in 18:12 for the Corinthian Jewish attack on Paul, and in 19:29 for the mob in Ephesus which rushed into the theater to (potentially) persecute Gaius and his companions. and 12:20 (political parties in agreement). It can even be used quite generically, as in Acts 5:12 where it simply has the sense of the word is simple “together.”
The idea of unity is important for Paul in his letters. In Phil 1:27, for example, he urges the readers to “be of one mind.” In Col 3:14 Paul tells his readers to “put on love” since that will bind them together in “perfect harmony.” There are examples in the Hebrew Bible of people coming together as “one person” (Ezra 3:1, Neh 8:1). The Greco-Roman world thought that harmony was a virtue, Dio Chrysostom said “For when we praise human beings, it should be for their good discipline, gentleness, concord, civic order, for heeding those who give good counsel, and for not being always in search of pleasures” (Or. 32.37).
In the first century, groups that were peaceful and harmonious were attractive, no one would join a movement which was perceived as fragmented and tempestuous. The reason that Luke highlights unity throughout the book is that unity and harmony where attractive to the Greco-Roman world. Luke presents the community in Jerusalem as unified around certain beliefs about Jesus as well as a few practices (prayer, sharing meals together, etc.)
If there is anything in the earliest community of followers of Jesus which ought to be a model for all churches, it is this unity of mind and purpose. It is fairly easy to point to church splits or denominationalism as symptoms of a larger problem. Everyone knows of a church which has been through a split, a pastor who was divisive, or a denomination which seems designed to fight with other Christians. It quite easy to point the finger at Fred Phelps (someone I do not consider a Christian). There are plenty examples of disunity in churches I respect.
But the early community did separate on some theological issues, primarily who Jesus was, and later in the book of Acts there will be significant differences between Paul and other Jews on the issue of Gentile salvation. While Acts 15 is sometimes used as a prime example of unity, there are some serious differences between the parties even in that point in story of Acts. This means that there are some things that are intensely important, things that unify Christians everywhere. It also means there are less-important issues. How can the church of the twenty-first century use the unity of the believers in Acts as a model for “doing church” today? What is it that ought to unify us, what might separate us?
My guess is that the things with unify are more important that the things which separate.