Because the Corinthians did not love one another, Paul gives them this teaching from himself, in order that they might learn to love each other with the same love with which they were loved by the apostle, not with carnal emotion but in Christ Jesus.
Thus to hinder them from thinking that in flattery to them he so ended, he saith, "In Christ Jesus." It having nothing in it human or carnal, but being of a sort of spiritual nature. Wherefore it is thoroughly genuine. For indeed the expression was that of one who loves deeply. As thus; because he was separated from them as regards place, as it were by the stretching out of a right hand he incloses them with the arms of his love, saying, "My love be with you all;" just as if he said, "With all of you I am." Whereby he intimates that the things written came not of wrath or anger, but of provident care, seeing that after so heavy an accusation he doth not turn himself away, but rather loves them, and embraces them when they are afar off, by these epistles and writings throwing himself into their arms.
For so ought he that corrects to do: since he at least, who acts merely from anger is but satisfying his own feeling; but he who after correcting the sinner renders also the offices of love, shows that those words also, whatsoever he spake in reproof, were words of fond affection.
[AD 384] Ambrosiaster on 1 Corinthians 16:24