{By this} (en tout(9369)). Locative case with en, "In this
way," viz., "if ye have love" (ean agap(886e) ech(8874)e), condition of
third class (in apposition with en tout(9369)) with ean and
present active subjunctive of ech(935c) ("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 
ot disciples (learners) of Jesus.