Soloveitchik was a life-long critic of all forms of non-Orthodox Judaism, including Reform Judaism and Conservative Judaism. He believed that these denominations were in significant error where they differed from Orthodox Judaism. He compared religious dialogue with Reform and Conservative leaders to dialogue between Pharisees and Karaites, considering it ridiculous. One of the major differences was in regard to the mixed seating in the synagogue. Consistent with the traditional rabbinic understanding of this issue, Soloveitchik ruled that it was forbidden to pray in a synagogue without a separation between the sexes (mi-d'orayta, a Pentateuchal prohibition), and without the use of a mechitza, a divider between the men's and women's sections (mi-derabbenan, a rabbinical prohibition). The effect of this was to prohibit prayer in any Reform synagogue and in many Conservative synagogues. His responsum on this question was also directed at the small number of Orthodox synagogues that were adopting mixed-sex seating. He was vociferous on this issue.