Perhaps Luke's most fundamental purpose in the Book of Acts is to help Christians answer the question "Who are we?" Two thousand years of church history sometimes prevents us from seeing just how basic that question was for the first believers. As long as Jews only were among the faithful, it could always be thought that this new group was just another sect of Jews who had some crazy notion about who the Messiah was. But as soon as Samaritans and Gentiles began entering the picture, identity with Judaism ceased to be an option. Something new had come into being - in continuity with the old, of course, but distinct from it as well. Luke, of course, leaves us in no doubt about whether the inclusion of Gentiles and the casting loose from temple and Torah were directed by God. And so a new name has to be coined to identify this new group: "Christians," followers of Christ (11:26).
An Introduction to the New Testament, Carson, Moo, pg. 325