Acts is the history of the development of the first churches and the spread of Christianity. For about the first decade after the death of Jesus, Christianity existed almost exclusively among the Jews. The first part of Acts portrays the meaning of Christianity, how it was shared, and how it was lived out in a closed community of Jews whose common culture was based on the Old Testament. After about the first decade, Christianity started growing in other groups and cultures that had beliefs and understandings quite different from the Jews. Much of Acts deals with how to keep a consistent and complete Gospel message, how it should be shared, and how it was lived out in different groups and cultures. The lessons learned from Acts are applicable to modern Christians and churches and are also essential when trying to understand Paul's letters (which comprise much of the New Testament). A purpose of the Eyewitness Bible Series: Acts is to tell some of the information of the book of Acts from different vantage points. Luke said that he made a careful investigation of the facts before writing Luke and Acts. His three primary sources of information probably were his own experiences, written sources, and talking to people who were personally involved. Eyewitness Bible Series: Acts illustrates the stories that he might have heard from a dozen of the characters featured in the book of Acts.