World Wide Study Bible
a Bible passage
21. If ye have heard him. To excite their attention and earnestness the more, he not only tells them that they had heard Christ, but employs a still stronger expression, ye have been taught in him, as if he had said, that this doctrine had not been slightly pointed out, but faithfully delivered and explained.
As the truth is in Jesus. This contains a reproof of that superficial knowledge of the gospel, by which many are elated, who are wholly unacquainted with newness of life. They think that they are exceedingly wise, but the apostle pronounces it to be a false and mistaken opinion. There is a twofold knowledge of Christ, — one, which is true and genuine, — and another, which is counterfeit and spurious. Not that, strictly speaking, there are two kinds; but most men falsely imagine that they know Christ, while they know nothing but what is carnal. In another Epistle he says,
“If any man be in Christ, let him be a new creature.”
(2 Corinthians 5:17.)
So here he affirms that any knowledge of Christ, which is not accompanied by mortification of the flesh, is not true and sincere.