I’ve been thinking about this poem a lot lately, because what it says could easily have been labeled heresy by the authorities of Saadi’s time, which was 13th century Iran, and an accusation of heresy could, conceivably, have gotten him killed. Not that Saadi believed Judaism was somehow equal in “truth” to Islam. Almost certainly, he believed the precise opposite. Still, the poem could be read as suggesting, negatively, that the two traditions have the same validity; and so it is important to note that Saadi is criticizing here the childishness of the people involved, not really making any claim about the relative value of the two religions. It’s the way the Muslim and the Jew view, respectively, Islam and Judaism that is the problem, not how each one feels about the other not following “the right and proper” faith:
Everyone thinks his own thinking is perfect and that his child is the most beautiful.
I watched a Muslim and a Jew debate
and shook with laughter at their childishness.
The Muslim swore, “If what I’ve done is wrong,
may God cause me to die a Jew.” The Jew
swore as well, “If what I’ve said is false,
I swear by the holy Torah that I will die
a Muslim, like you.” If tomorrow the earth
fell suddenly void of all wisdom
no one would admit that it was gone.
Edited to add that this is my translation.