Three things are required to be a disciple. The first is understanding, to grasp the words of the teacher: "Are you also still without understanding?" (Mt 15:16). But it is only Christ who can open the ears of the understanding: "Then he opened their minds so that they could understand the Scriptures" (Lk 24:45); "The Lord opened my ears" (Is 50:5). Secondly, a disciple needs to assent, so as to believe the doctrine of his teacher, for "The disciple is not above his teacher" (Lk 6:40), and thus he should not contradict him: "do not speak against the truth in any way" (Sir 4:30). And Isaiah continues in the same verse, "I do not resist." Thirdly, a disciple needs to be stable, in order to persevere. As we read above: "From this time on, many of his disciples turned back, and no longer walked with him" (6:67); and Isaiah adds: "I did not turn back" (Is 50:5). But it is a greater thing to know the truth, since this is the end of the disciple.
[St. Thomas Aquinas, Commentary on the Gospel of John, Chapters 6-12, Larcher, tr., CUA Press (Washington DC: 2010), p.124.]