Christianity is not a religion; it's all about Jesus and to be his children. The followers or the disciples were first called Christians in Antioch (Acts 11:26). God reveals his love for us in John 3:16 "For God so loved the world that He gave His one and only Son, that everyone who believes in Him shall not perish but have eternal life.
Spiritual growth shouldn't sound abstract to you because the God that we serve and believe in is spirit,so, when we talk about spiritual growth, it is all about becoming more like Jesus, and coming closer to God.