Adam Clarke on John 13:35

By this shall all men know, etc. - From this time forward, this mutual and disinterested love shall become the essential and distinctive mark of all my disciples. When they love one another with pure hearts, fervently, even unto death, then shall it fully appear that they are disciples of that person who laid down his life for his sheep, and who became, by dying, a ransom for all.

The disciples of different teachers were known by their habits, or some particular creed or rite, or point of austerity, which they had adopted; but the disciples of Christ were known by this love which they bore to each other. The primitive Christians were particularly known by this among the Gentiles. Tertullian, in his Apology, gives us their very words: Vide, inquiunt, ut se diligunt; et pro alterutro mori parati sunt. "See, said they, how they love one another, and are ready to lay down their lives for each other."