In this single verse (Mark 1:17 ESV), we find the three key elements of what it means to be a disciple:

1. Following Jesus (Being with Jesus, learning from Jesus, practicing the Way) > "follow me"  - It's relational and formational

2. Being Changed by Jesus (Becoming more like Jesus) >"and I will make you become" - It's transformational

3. Committed to Joining Jesus' Ongoing Mission in the World (Doing as Jesus did) >"fishers of men" - It's commissional

Using Pete Scazzero's definition of a disciple, we can expand on this definition:

• Following Jesus > We intentionally slow down to be with Jesus

• Being Changed by Jesus to Become More Like Him > We go beneath the surface of our lives to be deeply transformed by Jesus (we expectantly collaborate with Jesus for inner transformation, leading to lifestyle, outward change).

• Committed to Joining Jesus' Ongoing Mission in the World > We offer our lives as a gift to the world for Jesus 

Other definitions to the question about, "What is a disciple?"

• "...What is a disciple? In short, a disciple is a student. A disciple is one who disciplines himself in the teachings and practices of another. The word disciple, like discipline, comes from the Latin word discipulus, meaning “pupil” or “learner.” Consequently, to learn is to discipline oneself. For example, if one is to advance in the arts or the sciences or athletics, one has to discipline himself and to learn and follow the principles and fundamentals of the best teachers in that area of study. So it was and is with the disciples of Christ. A disciple follows Jesus." - Rev. Anthony Carter

• "A disciple is a follower, one who trusts and believes in a teacher and follows that teacher’s words and example. Therefore, to be a disciple is to be in a relationship. It is having an intimate, instructive, and imitative relationship with the teacher. Consequently, being a disciple of Jesus Christ is being in relationship with Jesus—it is seeking to be like Jesus." - Rev. Anthony Carter

• "The disciples of Jesus were with Him day and night for three years. They listened to His sermons and memorized His teachings. They saw Him live the life He taught. Then after His ascension, the disciples entrusted the word of Christ to others and encouraged them to adopt His lifestyle and obey His teaching.  A disciple is a student who memorizes the words, actions, and lifestyle of his teacher in preparation to teach others." - Keith Phillip

• "Throughout His ministry, Jesus made it clear to His listeners that being His disciples was not simply about receiving an education or even adhering to a set of ethical principles or stipulations. To be a disciple of Jesus meant to recognize Him for who He really was—the incarnate Son of God, the long-awaited Messiah—and therefore to reorient one’s life so that it conformed to the standards of His heavenly kingdom." - Greg Gilbert

• "Discipleship is about bringing all areas of our lives under the Lordship of Jesus by the power of the Spirit through the good news." - Seth McBee

• "A disciple of Jesus is someone who follows Jesus in order to become just like Jesus... someone whose life is centered on Jesus and the priorities he modeled and taught." - Justin Gravitt

• "A disciple of Jesus Christ is someone who is in the process of becoming like Jesus Christ in his/her affections, thoughts, and behavior." - Logan Gentry 

• “The distinguishing mark of Christian discipleship is a transformed heart, transformed affections. When someone becomes a true disciple, Christ radically changes the person’s appetite.”  - Eric Geiger, Michael Kelley, & Philip Nation

• “The supreme proof of true conversion is holy affections, zeal for holy things, longings after God, longings after holiness, desires for purity.” - Jonathan Edwards

• "Being a Christian is trusting Christ. Being a Christian is listening to Christ. Being a Christian is learning from Christ. Being a Christian is obeying Christ. Consequently, being a Christian is being a disciple. It was that way in the beginning. It continues to be that way today." - Rev. Anthony Carter

If you’re a Christian, you are a disciple of Jesus. And by necessity, a disciple undergoes discipline. If a disciple is a student, then discipline is training. Jesus’ discipline for you, however severe (and it is severe at times), is not God’s wrath against you. If you are tempted to believe that, don’t. It’s your unbelief or the enemy talking to you. When Jesus became sin for you (2 Corinthians 5:21), He removed all of sin’s condemnation from you (Romans 8:1).

No! Discipline is training. Training in what? Training in righteousness (2 Timothy 3:16). The unique training course that Jesus has designed for you (He designs a unique course for each disciple) has one great aim: to teach you to trust Him in everything. That’s His goal for you. Jesus wants you to learn to trust in Him in all things at all times. For the more you trust Jesus, the holier you become. 

- Jon Bloom