John 13:35

By this (en toutwi). Locative case with en, "In this way," viz., "if ye have love" (ean agaphn echte), condition of third class (in apposition with en toutwi) with ean and present active subjunctive of ecw ("keep on having love"). See 1 John 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 1 John 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 not disciples (learners) of Jesus.