Theophylact of Bulgaria
Again he begins to speak of vain arrogance, which is usually born of pride. As though bringing the discourse to a conclusion, he says that vainglory is evil. If it is evil, then it undoubtedly comes from the evil one.
But those who through holy Baptism have given themselves to the Lord must not accept what is sown by the evil one.
He also adds this: “To him who knows to do good and does not do it, to him it is sin.” Again he instructs the false teachers not to dare to teach what they have not first fulfilled themselves.
For it is said that blessed is not the one who teaches, but the one who does and teaches (Mt 5:19). Deeds must precede words, and by them the righteous man proclaims the faith he shows forth.
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For the Lord says: whoever breaks one of the least of these commandments and teaches men so, that is, teaches what he has not labored over himself, shall be called least. But whoever does and teaches shall be called great (Mt 5:19).
Therefore the God-man Himself first began to do, and then to teach (Acts 1:1).
In my opinion, the same thought is found in these words: “Let him who boasts boast in the Lord” (1 Cor 1:31). That is, boast in being like the Lord, having in Him both teacher and example.
In accord with this David also says: “My soul shall boast in the Lord” (Ps 33:2). He says nothing other than this: I would wish to boast that I walk according to the commandments of the Lord.