Albert Barnes Commentary Ephesians 4:21

"if so be that ye heard him, and were taught in him, even as truth is in Jesus:" — Ephesians 4:21 (ASV)

If indeed you have heard him. This means 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 you have heard him." See Barnes on Ephesians 3:2.

And have been taught by him. This could be by his Spirit or by the ministers whom he had appointed.

As the truth is in Jesus. This means you have learned the true nature of his religion as he himself taught it. What the truth was that the Lord Jesus taught, or what his principles implied, the apostle proceeds to state in the following verses.