A good reason to call oneself an "atheist" rather than "agnostic" is if you hold the opinion that existing religion must be actively challenged.
You may remember the "nullify the vegetarian moral crusade" campaign by Maddox:
"for every animal you don't eat, I'm going to eat three". This is similar in principle. Every time you observe the 458th mitzvot, I'm going to break it several million times.
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