Theophylact of Ohrid

And the words "as the truth is in Jesus" understand thus: you should not remain in the same state as the pagans walk. For Christ's work is not vanity, but truth, teaching, and life. Sin is a lie, for it lacks substance and has no purpose; but virtue is truth, for it is stable and has purpose.

Commentary on Ephesians ↗

Jerome

If all who seem to hear Christ did indeed hear him, the apostle would never had said this to the Ephesians. They were those to whom he had already revealed the promises of Christ. Why would he then say conditionally: "if indeed you have heard him?" To know Christ is the same thing already as knowing virtue. To hear of Christ rightly is the same as being attentive to all the virtues: wisdom, justice, temperance, fortitude and the other names by which Christ is called. …

Commentary on Ephesians 4:20 ↗

Thomas Aquinas

This certainly will happen if you have heard him, for hearing is the servant of instruction. "If" here is the equivalent of "because." "And this is the declaration which you have heard," as 1 John 1 (5) expresses it in reference to the proclamation of the faith. "He that hearkeneth to me shall not be confounded" (Ecclus. 24:30). And "the ear that heareth the reproofs of life shall abide in the midst of the wise. He that rejecteth instruction despiseth his own soul: but he that yieldeth to reproof possesseth understanding" (Prov. …

Commentary on Ephesians ↗