Commentary for Deuteronomy 29:18

והתברך בלבבו AND HE BLESS HIMSELF IN HIS HEART — The word והתברך has the meaning of “blessing”. In his heart he will imagine for himself a blessing of peace, saying, “These curses will not come upon me, I shall have peace”.

AND HE BLESS HIMSELF IN HIS HEART, SAYING: [‘I SHALL HAVE PEACE’]. When he will hear others imprecating themselves with these oaths, he will in his imagination bless himself, saying in his heart, “I shall have peace from any curse when I follow the desire of my heart.” G-d will not pardon him; instead, His anger will then be kindled against him²⁶Verse 19. whenever he sins, in that generation or [when his descendants sin] in later generations.²⁷This is to be understood in the light of Ramban’s explanation above, that the expression whose heart turneth away this day applies to one who has already been beguiled after the idols; or lest there should be among you an evil root refers to the descendants. It is with reference to these two types of idol-worshippers that Ramban says here that G-d’s anger will be kindled when he sins in that generation, or when his descendants sin in future generations (Kur Zahav). Or the sense of the verses may be as follows: “Because he says [publicly]: I shall have peace, since in my heart I do not accept the oath upon myself, for I shall walk in the stubborness of my heart all my days and carry out all my desires. And the Eternal will not be willing to pardon him²⁶Verse 19. because he did not accept His imprecation; instead, His anger will be kindled against him²⁶Verse 19. since he came into the covenant in My presence with all [the people of] Israel.” [Therefore the refusal of an individual to accept the covenant constitutes a punishable defiance.]
A desire that grows stronger and more in one’s heart is termed [in Hebrew] shriruth [‘ki bishriruth libi eileich’ — for I shall walk in ‘the stubborness’ of my heart], similar to the expression of the Sages:²⁸Baba Bathra 160b. “sharir (strong) and established.” So also, ‘bishrirei’ (in the muscles of) his body.²⁹Job 40:16.

והתברך בלבבו, while accepting the covenant with his lips, he already resolves in his heart to honour it in the breach, comforting himself, thinking שלום יהיה לי כי בשרירות לבי אלך, “I will be alright even though I will do my own thing.” He considers that seeing that he already planned to breach the covenant at the very moment when he accepted it with his lips, the covenant is not binding upon him.