There is one thing which, when it appears in the world, appears for the detriment of many, for the misery of many, for the loss, harm, and suffering of many. What is that one thing? It is disunity in the Sangha.
When the Sangha is divided there are mutual quarrels, mutual recriminations, mutual denigrations, and mutual rejections. In this situation, those who are unsympathetic to the Dharma remain unsympathetic, and some who are sympathetic change their minds.
—Buddha Shakyamuni, Sanghabheda Sutta