Albert Barnes on Ephesians 4:21

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." Eph 3:2.

And have been taught by him. By his Spirit, or by the ministers whom he had appointed.

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