Theophylact of Bulgaria
According to the Jewish understanding, the pious man is one who shows fidelity in works, because such a man seems not to belong to the crowd.
The Jews, while fulfilling the observances prescribed by the law, thought highly of themselves. They concentrated all piety toward God in these observances.
Being occupied with these alone, they imagined that through them they would acquire blessedness. They treated others with great condemnation, as is clear from the Gospel parable of the boastful Pharisee and the publican (Lk 18:10-14).
Restraining them from such an opinion, the Apostle gives the present instruction. Having mentioned the doer of the work and called him blessed, he at once corrects the evil that arises in many during fulfillment.
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He speaks as if saying: "You who boast in fulfilling the law, do not think that you will receive blessedness for fulfillment alone."
For this is not pleasing to God. Rather, pleasing to Him is the man who fulfills and, at the same time, is far from conceit and does not condemn those who do not fulfill.
"Deceives his heart" means that he, as it were, chokes it, and through conceit, as a doer of the law, deceives his conscience.
Here "heart" means the same as in the words, "A broken and humbled heart" (Ps 50:19).