A.T. Robertson Commentary John 13:35

"By this shall all men know that ye are my disciples, if ye have love one to another." — John 13:35 (ASV)

By this (εν τουτω). Locative case with εν, "In this way," viz., "if ye have love" (εαν αγαπην εχητε), condition of third class (in apposition with εν τουτω) with εαν and present active subjunctive of εχω ("keep on having love"). See 17:23 where Jesus prays for mutual love among the disciples "that the world may know" that the Father sent him. Jerome (ad Galat. vi. 10) says that in his extreme old age John repeated often this command of Jesus and justified it: "Because it is the Lord's commandment; and if it be fulfilled it is enough." See also 14:31. Tertullian (Apol. 39) urges it also as proof of being disciples. Hatred of one another per contra, is an argument that we are νοτ disciples (learners) of Jesus.