Being a Christian is simply being a disciple of Jesus Christ.
The word "disciple" means learner, student, pupil, or follower. To be a disciple means that we learn from and follow Jesus.
To fill this out, it's helpful to look at what Jesus himself said when he enlisted his first disciples.
Mark 1:16-20 (NIV)
As Jesus walked beside the Sea of Galilee, he saw Simon and his brother Andrew casting a net into the lake, for they were fishermen. "Come follow me," Jesus said, "and I will make you fishers of men." At once they left their nets and followed him.
When he had gone a little farther, he saw James son of Zebedee and his brother John in a boat, preparing their nets. Without delay he called them, and they left their father Zebedee in the boat with the hired men, and followed him.
Five important things can be said about this passage.
An enduring relationship
"Follow me", he says.
Jesus invites these men to enter into an enduring relationship with him.
Following would mean traveling with him, working with him, eating with him. They would journey with him as he ministered in Galilee and beyond. In short, they would share his life.
This is not a one-time "transaction of faith". It is an enduring friendship.
A transformation of character
"and I will make you . . ."
Jesus promises a transformation in their lives, a transformation of character and identity.
This change does not happen magically or instantaneously. It occurs over time, as the teachings of Jesus slowly soak in, and from simply observing him, spending time with him.
Being a disciple does not result merely in a change of status before God (from "lost" to "saved", or "guilty" to "pardoned"). It also results in a transformation of character flowing from an enduring relationship with Christ.
All relationships change us. If there is no change, there is no relationship.
A social experience
"and I will make you (the two of you) . . ."
It is interesting to note that Jesus approaches two sets of brothers to join him as his first disciples. If they accepted his invitation, not only would they be drawn into relationship with him, they would be drawn into relationship with one another. A little later, Jesus would invite a notorious tax collector (Matthew) to join the group, something the fishermen probably hadn't counted on when they signed up.
Those on a journey with Jesus cannot help rubbing shoulders with others who are also on a journey with him.
The New Testament knows next to nothing of Christianity as a private experience. Faith is lived out in the social context of other believers: the church.
A life of mission
" . . . fishers of men."
Jesus was on a mission to bless the world. He would do so by teaching, by healing the sick, by accepting the outcasts, and, eventually, by dying on the cross.
The disciples he enlisted would join him in this mission, and they, in turn, would enlist others to do the same.
Being engaged in a life of missionary service does not mean that one must move to Africa or China. It does mean that, wherever we live and whatever our jobs, we must bless others as Jesus would if he were in our place.
We are not disciples of Jesus for our own benefit. Disciples continue the ministry of Jesus, blessing others in his name.
Marked by repentance and sacrifice
"At once they left their nets and followed him."
If we set out on a trip, we cannot take everything we own. Some of our stuff simply won't fit in the luggage.
One of the saddest stories of the New Testament is the story of the "rich young ruler" (Mark 10:17-31). He didn't follow Jesus because he wasn't willing to leave anything behind.
If we set out on a journey with Jesus, we have to leave some of our "stuff" behind. For some of us, it is a sinful habit of mind or action that must be discarded. For others, it is an unwise relationship, or a dishonorable career. For still others, it may be an enduring grudge.
But we all leave something behind, if we want to follow Jesus. And, while we're on the road, we can't pick up everything we see, either.
There are sacrifices we make to stay on the journey. What we do with our time, our money, our careers, our relationships is all affected by the choice we make to become a disciple of Jesus.
What it means to be a Christian
A Christian follows Jesus.
A follower is one who enters an enduring relationship with Jesus, is changed by that relationship, is involved in a community of faith with other disciples, is living a life of mission in harmony with the mission of Jesus, and has (or is in the process of) discarding everything in his/her life that is out of step with Jesus.