How do you tell if a tree is alive or dead? How do you tell if a person who professes to be a Christian is spiritually alive? In John 15 Jesus used a simple metaphor for the Christian life, the grape vine and its branches, to describe the characteristics and the responsibilities of His followers. This is one of the most important teachings on how to live the Christian life. It is also a “wake up” call to people who profess their faith in Christ but whose lives are spiritually barren.