To be a disciple is one of the most obvious words used to describe the early followers of Christ. We might not use it much today, but it is still a good description of what it means to follow Jesus.
Since the word disciple has only really a religious or philosophical use in English, dictionary definitions of the word disciple are fairly straightforward.
a person who believes in and follows the teachings of a religious or political leaderwww.oxfordlearnersdictionaries.com [accessed 8 January, 2020]
a convinced adherent of a school or individualwww.merriam-webster.com [accessed 8 January, 2020]
Perhaps the most famous use of the word disciple is the action (verb) version of the word used in the Great Commission where the original disciples of Jesus were commanded to make disciples of all the nations. The verb in Matthew 28:19-20 for “make disciples” is the Greek word μαθητεύω, or, with English letters, mathēteúō.
Mathēteúō is a specialised form of learning from another just like the English definition of disciple is more specialised than the word student. If I start teaching political ideals on the street corner, I might gain a few students, but they are only rightfully called my disciples when they corner me afterwards and together we start a political party based on those political ideals. That very active sense of learning, believing, following, and associating is what makes mathēteúō the word of choice in the Great Commission from our Lord Jesus.
To be made a disciple of Jesus Christ is not just to sit in Sunday School. To be made a disciple of Jesus Christ is to have your hearing of the Word of God turned into faith (Rom 10:17). To be made a disciple of Jesus Christ is to call him Lord and to do what He teaches (Luk 6:46-49). To be made a disciple of Jesus Christ is to have the seed sown in your heart and produce much fruit (Luk 8:9-15).
Christians are not just students learning stuff. Christians are specialised students of a very specific individual. To be a Christian is to learn the truth of Jesus Christ, to believe the truth of Jesus Christ, and to obey the truth of Jesus Christ.