The modern ear will immediately recognize this argument. Faith is a matter of conscience, not a matter of law, and it is unfair to expect those who do not truly profess the faith to be bound by its strictures. We have been taught to even admire one who sticks to his principles, even if we disagree with them. This then is the argument that one can do no wrong if one is true to the dictates of one's heart. Our individual's blithe self-confidence is based on the obverse of Moshe's concern above.
Actions are ultimately the expression of our inner commitments.
Moshe knows that without such a commitment, the expressed acceptance of the berit will not endure. Similarly, it may be argued that they have no reason to endure, no validity. The Torah is warning that one who follows his heart will degenerate, his heart spinning out of control beyond what he originally desired. And God shall separate him from all the tribes of Israel, according to all of the curses of the covenant, written in this book of the Torah. By My life, says HaShem the God, I shall rule over you with a mighty hand, an outstretched arm, and with anger poured out.