Sforno on Deuteronomy 29:18/ספורנו על דברים כ״ט:י״ח

1והתברך 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.

2The 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.