Deuteronomy 29:18

1. והתברך בלבבו. This refers to blessing. The person in question will bless himself as if to say, "These curses will not come to me, rather, I shall have peace."

2. והתברך. In Old French, this means benir soi, or bless oneself, as in התגלח (to shave oneself) and התפלל (pray, literally to judge oneself).

3. בשררות לבי אלך. I shall do as my heart sees fit.

4. למען ספות הרוה. "For I (God) shall add to his punishment for what he has already done by accident, which I ignored. He now causes me to add to them his deliberate sins, and I will punish him for everything." Thus is how Onkelos translates it.

5. הרוה - "Moist" refers to sins that are committed by a drunkard, without intention.

6. הצמאה - "Thirsty" refers to one who sins deliberately and with desire.

or it be that when he heareth the words of this curse he become reprobate in his heart, saying: I shall have peace, though I go on in the strength of the evil desires of my heart: so that he will add presumption to the sins of ignorance.

this curse “Cursed be the man who makes a graven or molten image…” [27: 15].

bless himself Rabbi Yehudah HaLevy, of blessed memory, said that the word “bless himself” should be taken literally: When he hears the malediction, in his heart he will consider himself blessed, thinking all will be well with me. In other words, he will hear the ban, but think to himself, “I am an exception”.

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 him whenever he sins, in that generation or [when his descendants sin] in later generations. 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 him because he did not accept His imprecation; instead, His anger will be kindled against him 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: “sharir (strong) and established.” So also, ‘bishrirei’ (in the muscles of) his body.

למען ספות הרוה, the deliberate sinner, whose motivation is that seeing he is so well off, he does not worry about possible consequences for himself

הצמאה, because of the urgings of his desire. He does not sin in order to make a statement against G’d asserting his freedom to do as he pleases and thereby rejecting G’d as the world’s Master. He is merely unable to resist his urges.

כי בשרירות לבי, only because of assertion of self, of ego.

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.

The only reason he had mouthed acceptance of the covenant was to become at one and the same time a beneficiary of the overwhelming number of people for whose sake G’d was going to bestow so many blessings on His people. This is the meaning of the words שלום יהיה לי כי בשרירות לבי אלך למען ספות הרוה את הצמאה; he knows full well that the holy community of the Jewish people, his brethren, have attained that status by resisting the lures of the evil urge instead of succumbing to it. This is why they will benefit from G’d’s largesse in such abundant manner. He therefore decided to have the best of both worlds by at least paying lip service to a covenant he knows he will not live by.

והיה בשמעו את דברי האלה הזאת וגו, “and when he hears the wording of this curse [which appeared to him as addressed to the community, as opposed to individuals in the community, Ed.] והתברך, “and he will personally feel immune in his heart;” שלום יהיה לי, “I, personally will be quite safe;” בשרירות לבי אלך, ”and therefore I will follow whatever my heart dictates to me;” למען ספות הרוה את הצמאה, “so that the irrigated will be swept away with the dry;” [the sinner’s logic is that on account of one single sinner G–d will not withhold blessings such as rain from a whole community. Ed.] Alternate meaning, the author’s: “he will commit sins that are not the result of the evil urge as well as sins that are the result of his evil urge.” לא יאבה ה' סלוח לו, כי אז יעשן אף ה' וקנאתו, “not only will the Lord not consider forgiving him his foolishness, but His anger and jealousy will be kindled against such an individual and He will single him out for especially harsh punishment. By doing this the remainder of you will be shielded against copying his conduct, and you will remain loyal to G–d’s covenant. For if you collectively would fail to remain loyal ורבצה בו כל האלה, “the Lord will blot out his name,” so that no descendants of such a sinner will remain alive under the sky. Neither an already an existing grandson or great-grandson will be allowed to live. והבדילו ה' לרעה, “and the Lord will single him out for misfortune; that sinner had thought that he alone of all the Israelites could get away with being disloyal to the covenant. I will show the Israelites that he alone will be singled out for such harsh treatment by Me. The punishment will fit the crime.