Paul says Christ teaches us to be good (Eph 2:10).
“If so be that ye have heard him [Christ], and have been taught by him [Christ]…” – Eph 4:21
Some might think this means we should go back to the Sermon on the Mount to learn what Christ taught, but this would be unwise.
When Jesus taught the Sermon on the Mount he was speaking to Israel under the law, not grace (Rom 15:8, Matt 15:24).
He taught the law, conditional forgiveness, covenants with God, selling all your possessions, and not working for your food.
To Israel under the law, Christ taught to go back to the Moses and the fathers, straighten up and fly right, and the prophecies for the kingdom and salvation would be fulfilled (Matt 6:33).
Christ taught Israel could know the good branches by their fruit. If they did not bear fruit for the kingdom, then they would not enter the kingdom, but be cut off (Matt 7:16-21).
No, Paul does not preach the gospel of God’s grace only to place us back under the law to help us “be good”.
Rather, Paul refers to the special dispensation of grace (Eph 3:2). Now, Christ teaches the church through truth revealed to Paul for us (Rom 15:16). Christ teaches the church how to be good under grace and not law (Rom 6:14).
What Christ Teaches the Church
By grace Christ teaches us to put off the old man, to renew our minds, and to put on the new man created in Him (Eph 4:22-24).
These lessons to the church are instructions for people who have already trusted the gospel of salvation, who are already saved. It is impossible to put off the old man without Christ first dying for our sins and crucifying our old man (Rom 6:6).
Under law, the fruit was the sign of their kingdom admission. If someone was not bearing fruit under the law then there was a real problem of going into the kingdom. Without fruit they were not one of the kingdom saints.
Under grace, the only sign of our admission into the church is nothing but the blood of Jesus – the fruit of the Lord Jesus Christ.
If someone is not bearing fruit under grace then there is a problem applying the three principles Christ teaches in Eph 4:21-24.
When someone learns what Christ teaches by grace, they will bear fruit. We can know by the fruit of the Spirit if someone has put on the new man or is still submitting to their old man.
The unsaved person is not a new man. The saved person has a choice to know both his old man and the new man. We ought to reckon the old dead, and live in the new.
This is the crux of what Paul is saying to the Ephesians concerning people saved by grace living like unbelievers:
“But ye have not so learned Christ;” – Eph 4:20
Christ teaches us to be good, but to be good under grace not law (Titus 3:8).