The answer is simple: We are right and they are wrong. We were told by G-d to do so and they were not (despite claiming to be doing G-d’s will.) We can criticize them because they are going against G-d’s will whereas we are going and went in accordance with G-d’s will. Our argument is simply we are doing G-d’s will whereas they are not. Whether it is provable or unprovable to non-Jews is secondary. We don’t need to have a winning argument in order for us to speak and know the truth.
