John Calvin 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 distinguishing between his disciples and strangers, they who lay aside brotherly love, and adopt new and invented modes of worship, labor in vain; and folly of this kind prevails at this day in Popery. Nor is it superfluous that Christ dwells so largely on this subject. There is no greater agreement between the love of ourselves, and the love of our neighbor, than there is between fire and water. Self love keeps all our senses bound in such a manner that brotherly love is altogether banished; and yet we think that we fully discharge our duty, because Satan has many enticements to deceive us, that we may not perceive our faults. “A ce que nous n’appercevions nos fautes.” Whoever, then, desires to be truly a disciple of Christ, and to be acknowledged by God, let him form and direct his whole life to love the brethren, and let him pursue this object with diligence.

John 13:36-38

36. Simon Peter saith to him, Lord, whither goest thou? Jesus answered him, Whither I go, thou canst not follow me now; but thou shalt follow me afterwards. 37. Peter saith to him, Lord, why cannot I follow thee now? I, will lay down my life for thee. 38. Jesus answered him, Wilt thou lay down thy life for me? Verily, verily, I tell thee, The cock will not crow until thou hast denied me thrice.