{4} Then Peter, turning about, seeth the disciple whom Jesus loved following; which also leaned on his breast at supper, and said, Lord, which is he that betrayeth thee?

(4) We must take heed that while we cast our eyes upon others that we do not neglect that which we are commanded to do.

Wesley's Notes

21:20 Peter turning - As he was walking after Christ. Seeth the disciple whom Jesus loved following him - There is a peculiar spirit and tenderness in this plain passage. Christ orders St. Peter to follow him in token of his readiness to be crucified in his cause. St. John stays not for the call; he rises and follows him too; but says not one word of his own love or zeal. He chose that the action only should speak this; and even when he records the circumstance, he tells us not what that action meant, but with great simplicity relates the fact only. If here and there a generous heart sees and emulates it, be it so; but he is not solicitous that men should admire it. It was addressed to his beloved Master, and it was enough that he understood it.

Jamieson-Fausset-Brown Bible Commentary

20, 21. Peter, turning about-showing that he followed immediately as directed.

seeth the disciple whom Jesus loved following; which also leaned on Jesus' breast at supper, and said, Lord, which is he that betrayeth thee?-The Evangelist makes these allusions to the peculiar familiarity to which he had been admitted on the most memorable of all occasions, perhaps lovingly to account for Peter's somewhat forward question about him to Jesus; which is the rather probable, as it was at Peter's suggestion that he put the question about the traitor which he here recalls (Joh 13:24, 25).

Matthew Henry's Concise Commentary

21:20-24 Sufferings, pains, and death, will appear formidable even to the experienced Christian; but in the hope to glorify God, to leave a sinful world, and to be present with his Lord, he becomes ready to obey the Redeemer's call, and to follow Him through death to glory. It is the will of Christ that his disciples should mind their own duty, and not be curious about future events, either as to themselves or others. Many things we are apt to be anxious about, which are nothing to us. Other people's affairs are nothing to us, to intermeddle in; we must quietly work, and mind our own business. Many curious questions are put about the counsels of God, and the state of the unseen world, as to which we may say, What is this to us? And if we attend to the duty of following Christ, we shall find neither heart nor time to meddle with that which does not belong to us. How little are any unwritten traditions to be relied upon! Let the Scripture be its own interpreter, and explain itself; as it is, in a great measure, its own evidence, and proves itself, for it is light. See the easy setting right such mistakes by the word of Christ. Scripture language is the safest channel for Scripture truth; the words which the Holy Ghost teaches, 1Co 2:13. Those who cannot agree in the same terms of art, and the application of them, may yet agree in the same Scripture terms, and to love one another.