John 13:35 (BSB)

By this everyone will know that you are My disciples, if you love one another.”

From John 13. Also in the ESV.

Commentary on John 13:35

• John Calvin (Reformed), Calvin's Commentaries on John 13:35: 35. By this all men will know. Christ again confirms what he had formerly said, that they who mutually love one another have not been in vain taught in his school; as if he had said, Not only will you know that you are my disciples, but your profession will also be acknowledged by others to be sincere.” Since Christ lays down this mark for...
• Matthew Henry (Presbyterian), Matthew Henry's Complete Commentary on John 13:31-35: This and what follows, to the end of ch. 14, was Christ's table-talk with his disciples. When supper was done, Judas went out; but what did the Master and his disciples do, whom he left sitting at table? They applied themselves to profitable discourse, to teach us as much as we can to make conversation with our friends at table serviceable to religion. Christ begins this discourse.
• John Gill (Reformed Baptist), Exposition of the Old and New Testaments on John 13:35: By this shall all men know,.... Not only by this you yourselves will know that ye have passed from death to life, that the true work of grace is begun upon your hearts; nor only by this will you know one another to be Christians; but by this all men, even the men of the world will know, that ye are my disciples, if ye...
• Albert Barnes (Presbyterian), Barnes' New Testament Notes on John 13:35: Verse 35. By this shall all men, &c. That is, your love for each other shall be so decisive evidence that you are like the Saviour, that all men shall see and know it. It shall be the thing by which you shall be known among all men.