From the beginning of human history down to the present time the various religions of the world have anathematized and accused each other of falsity. Each religion has considered the others bereft of the face of God, deprived of His mercy and in the direct line of divine wrath. Therefore, they have shunned each other most rigidly, exercising mutual animosity and rancor. Consider the record of religious warfare, the battles between nations, the bloodshed and destruction in the name of religion. One of the greatest religious wars, the Crusades, extended over a period of two hundred years. In this succession of great campaigns the western crusaders were constantly invading the Orient, bent upon recovering the Holy City from the hands of the Islamic people. Army after army raised in Europe poured its fanatical legions into the East. The kings of European nations personally led these Crusades, killing and shedding the blood of the Orientals. During this period of two hundred years the East and West were in a state of violence and commotion. Sometimes the crusaders were successful, killing, pillaging and taking captive the Muslim people; sometimes the Muslims were victorious, inflicting bloodshed, death and ruin in turn upon the invaders. So they continued for two centuries, alternately fighting with fury and relaxing from weakness, until the European religionists withdrew from the East, leaving ashes of desolation behind them and finding their own nations in a condition of turbulence and upheaval. Hundreds of thousands of human beings were killed and untold wealth wasted in this fruitless religious warfare. How many fathers mourned the loss of their sons! How many mothers and wives lamented the absence of their dear ones! Yet this was only one of the “holy” wars. Consider and reflect.
— ‘Abdu’l-Baha, The Promulgation of Universal Peace, p. 265