While living in Philadelphia, Mendes Seixas zealously defended religious liberty. He and several members of his congregation addressed the Commonwealth of Pennsylvania's Council of Censors in December 1783, opposing the adoption of a religious test for prospective office holders. He characterized these as "unjust to the members of a persuasion that had always been attached to the American cause and given a support to the country, some in the Continental army, some in the militia, and some by cheerfully paying taxes and sustaining the popular cause."