An atheist told me that rather than being the Word of God, the Torah and the Mitzvot were actually concocted by the Rabbis and therefore they are not binding.
According to his own admission, all mitzvot, being invented by rabbis, are d'rabbonon. But even a mitzva d'rabbonon is binding.
Can anyone resolve this apparent contradiction?
The fact that even a mitzva d'rabbonon is binding stems from a mitzva in the torah -
עַל פִּי הַתּוֹרָה אֲשֶׁר יוֹרוּךָ וְעַל הַמִּשְׁפָּט אֲשֶׁר יֹאמְרוּ לְךָ תַּעֲשֶׂה לֹא תָסוּר מִן הַדָּבָר אֲשֶׁר יַגִּידוּ לְךָ יָמִין וּשְׂמֹאל
“According to the law that they direct you and the judgement that they will say to you you should do, you should not stray from the matter that they instruct you to the right or to the left.”
So the Rabbis told you to listen to the Rabbis. So every mitzva is a גזירה לגזירה.
The gemara in Bava Basra 60b states
שאין גוזרין גזרה על הצבור אלא אם כן רוב צבור יכולין לעמוד בה
Any decree which the public cannot handle should not be made
Ritva there:
ואם גזרו אינה גזירה כלל
And if they do make such a decree, it is not valid
Every mitzvah is a גזירה שאין הציבור יכול לעמוד בו, as we see from the fact that he isn't doing them. So they aren't binding.
By "not binding" he meant that he believes that violating them does not subject one to the death penalty, imposed in some cases by tying a rope around the guilty party's neck.
