The answer is: Luke

Acts 1:1 / Luke 1:3 - showing Acts to be a subsequent work of Luke.

The Book of Acts, often dubbed “the sequel to Luke,” perfectly illustrates the expansion of early Christian communities after Jesus' resurrection. Written by Luke, a physician and one of Paul’s travel companions, it offers a compelling narrative about the apostles’ adventures, highlights the struggles they faced, and intimately portrays the growth of the Church.

Fun fact—you'll notice that Luke displays a significant flair for storytelling! He has a knack for using vibrant descriptions and character-driven narratives to capture the vital emotions and dynamics of its key figures. Because of this, many consider Luke’s writing to be the most literary of the Gospels. Plus, he's the only Gospel writer who narrates parables like the Good Samaritan and the Prodigal Son.

Importantly, Acts doesn’t shy away from trials and persecution faced by the early Christians, engaging us with more than just miraculous events. It addresses themes of loyalty, faith, and the occasionally funny—albeit tense—moments between competing communities.

Consider the account of Peter in Jerusalem and Paul’s mission trips—a genuine reflection on how the early followers didn’t find success handed to them on a silver platter. It’s rich with lessons in courage and resilience.

Ultimately, Acts serves not just as a record of history but a call to action, reminding readers that the spirit of these early followers is alive and well within each quest for understanding and practice of moral and ethical living in today’s diverse world.