Rabbi Moshe Feinstein's influential legal rulings systematically excluded Conservative and Reform Judaism from legitimate religious authority, despite his reputation for personal kindness and halakhic leniency. Through analysis of his responsa collection *Igrot Moshe*, particularly volumes from 1973 and 1985, a clear pattern emerges of rigid boundaries between Orthodox and non-Orthodox movements. The methodology involves close textual examination of specific responsa concerning conversion, marriage, divorce, witness qualification, and institutional relationships between Orthodox and non-Orthodox communities. Feinstein categorically disqualified Conservative and Reform rabbis as witnesses, invalidated their religious procedures including conversions and marriages, and prohibited Orthodox institutional cooperation with non-Orthodox movements. Mere membership in Conservative or Reform rabbinates constituted sufficient grounds for disqualification, regardless of individual observance levels. These rulings reflect a systematic rejection of religious pluralism within Judaism, with Feinstein applying the principle of "letting the law penetrate the mountain" to create comprehensive barriers between Orthodox and non-Orthodox communities. The research concludes that bridging this halakhic divide requires more than improved interfaith relations, as the fundamental methodological differences regarding Jewish law and authority remain irreconcilable within Feinstein's Orthodox framework.