Ephesians 4:21

If so be that ye have heard him, and have been taught by him, as the truth is in Jesus:

If so be that, [ ei-ge (G1489), with indicative] - Assuming what I have no reason to doubt, that, etc.

Heard him. The Him is emphatic: heard Himself, not merely about Him (John 10:27).

Taught by him - Greek, 'taught IN Him;' i:e., in vital union with Him (Romans 16:7): the further instruction, besides having "heard Him" at conversion.

As the truth is ... Connect with "taught:" 'And in Him, have been taught, according as is truth in Jesus.' There is no article. "Truth" is truth in its essence and perfection in Jesus; 'if according as it is in Him, ye have been so taught in Him:' in contrast to 'the vanity of mind of the Gentiles' (cf. John 1:14; John 1:17; John 18:37; Ephesians 4:17). Knowledge of Christ not accompanied with 'putting off lusts' is not true, but spurious. 'What in Jesus is truth, not semblance, is to become truth also in the faithful' (Olshausen). Contrast John 8:44.