if indeed you heard him, and were taught in him, even as truth is in Jesus:
21. If ye have heard him. To excite their attention and earnestness the more, he not only tells them that they had heard Christ, but employs a still stronger expression, ye have been taught in him, as if he had said, that this doctrine had not been slightly pointed out, but faithfully delivered and explained. As the truth is in Jesus.
Verse 21. If so be that ye have heard him. If you have listened attentively to his instructions, and learned the true nature of his religion. There may be a slight and delicate doubt implied here whether they had attentively listened to his instructions. Doddridge, however, renders it, "Seeing ye have heard him." . And have been taught by him.
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 (Rom 16:7).