My
personal definition of a christian is
"Someone who attempts to follow the teachings of Jesus"
This definition is a broad one and incorporates those who go to church every Sunday and sing in the choir, to those who have never been in a church in their lives but feel that the teachings of Jesus make sense
The essence of what Jesus taught he summed up as follows:
"You must love the Lord with all your heart, mind and soul. This is the first and greatest commandment. A second equally important commandment is that you love your neighbour as yourself. All the other commandments and all the demands of the prophets are based on these two commandments." (Matthew 22:37-40)