The book is about the relationship between Jesus and his followers(the Christians). Jesus himself describes this relationship with the analogy of a Vine and its branches. Jesus himself is the Vine while Christians are its branches. As the branches of a vine derive their lives from the vine and are kept alive by the vine so Christians derive their lives from Jesus and are kept alive by Jesus as long as they remain in union with him. As the vine and its branches are one, so Jesus and Christians are one. As the branches of a vine live in the vine, with the vine, and for the vine, so do Christians live in Jesus, with Jesus, and for Jesus. As any branch that is cut off from the vine will wither and die so will any Christian who cuts himself/herself off from Jesus by mortal sin die. Mortal sin kills because it cuts a person off from Jesus, the source of life, Hence to commit mortal sin is to commit spiritual suicide. The Divine Vine divinizes its branches by elevating them to the level of being able to share in the Divine life and the Divine nature, as Sacred Scripture says. What a wonderful privilege to be a Christian and live in union with Jesus as a branch lives in the Divine Vine.