John Starke posts Give Up Your Weak Definition of 'Disciple':
Sermons, talks, and books on discipleship usually give a basic definition of disciple as "learner." But the New Testament gives us a more thrilling and dynamic definition of a disciple and the cost that follows. Take for example the parable of the soils in Matthew 13. How do we know a disciple from merely a "learner"? Matthew 13:23 says, "He indeed bears fruit and yields, in one case a hundredfold, in another sixty, and in another thirty." A disciple is, by nature---by definition!---a multiplier.
Disciples are not merely learners but fruit-bearing disciple-makers; they multiply themselves.
As you read the New Testament, you see that discipleship is complex and thrilling. Hans Kvalbein wrote in 1988 a Themelios article on the concept of discipleship in the New Testament (see the entire archive of Themelios articles) that gives strength and depth to how local churches should think, talk, and teach about discipleship. He gives 13 theses on discipleship. Here are several of them in summary form:
The first word for Christians was not "Christians" but "disciples." Newcomers to the faith saw themselves in relationship to the risen Lord Jesus in some way similar to the relationship of the first disciples to the earthly rabbi Jesus.
A disciple learns by hearing his Master and doing like his Master. This is explicitly commanded by Jesus in John 13: "You call me 'Teacher' and 'Lord' and rightly so, for that is what I am. Now that I your Lord and Teacher have washed your feet, you also should wash one another's feet. I have set an example that you should do as I have done for you."
The disciples are chosen by Jesus. He says, "You did not choose me, But I chose you to go and bear fruit" (John 15:16).
Life as a disciple is "death and resurrection" with Christ, inaugurated in Christian baptism. Baptism is initiation into discipleship, giving admittance to the "school" of Jesus and starting a new life in obedience to him and his commands.
To be a disciple is to be called to make new disciples. Throughout the New Testament, the term "disciple" is a dynamic concept. It implies multiplication. All nations have been invited to this mission. "He is the atoning sacrifice for our sins, and not only for ours but also for the sins of the whole world" (1 John 2:2).
You can read the rest of Kvalbein's helpful article for the entire list and helpful summaries.
As we should expect, whenever the New Testament explains discipleship, it immediately warns us of the cost. Being a follower of Jesus splits up your family, threatens your life, and calls you to radical sacrifice of your job, finances, desires, hopes, and reputation.
However, the cost comes after a promise. When Jesus tells his disciples to "sell your possessions, and give to the needy" (Luke 12:32), he does so after a very important assurance: "Fear not, little flock, for it is your Father's good pleasure to give you the kingdom." In Jesus' day, they didn't have banks, savings, or CDs. Their net-worth was found in their possessions. Jesus is saying, if you are my disciple, you should be able to radically give, even if it means dipping into your savings (see Joel Green's The Gospel of Luke).
But notice the order. Jesus doesn't say, "Sell everything, give to the poor, and God will give you his kingdom." It's the exact opposite. Disciples can be radical in their giving because they've already been given the kingdom. Christ became poor, so his disciples might become rich (2 Cor. 8:9), and out of infinite riches, disciples become generous.
Another example is when Jesus instructs his disciples that he will be sending them out as "sheep in the midst of wolves," and the wolves will "deliver you over to courts and flog you in their synagogues" (Matthew 10:16-17). That's not a very encouraging rallying cry for church growth and evangelism! Notice that Jesus isn't saying that they will be suddenly attacked by wolves; he's sending them out into them. This is deliberate. They will be mocked, shamed, and possibly even killed. He knows this will happen to them, because that's what the wolves will do to him. No servant is greater than his Master. Only when disciples are filled with God's grace can they go into the wolves hoping to see some turn into lambs.
Discipleship is as dynamic and costly as the gospel. Disciples can give up anything, go anywhere, and risk everything because the gospel has filled us with good things that can never be taken away.