21. If so be that--not implying doubt; assuming what I have no reason to doubt, that
      heard him--The "Him" is emphatic: "heard Himself," not merely heard about Him.
      taught by him--Greek, "taught IN HIM," that is, being in vital union with Him (Ro 16:7).
      as the truth is in Jesus--Translate in connection with "taught"; "And in Him have been taught, according as is truth in Jesus." There is no article in the Greek. "Truth" is therefore used in the most comprehensive sense, truth in its essence, and highest perfection, in Jesus; "if according as it is thus in Him, ye have been so taught in Him"; in contrast to "the vanity of mind of the Gentiles" (Eph 4:17; compare Joh 1:14, 17; 18:37). Contrast Joh 8:44.

JFB.