Religion is the masterpiece of the art of animal training, for it trains people as to how they shall think.

Let us see rather that like Janus—or better, like Yama, the Brahmin god of death—religion has two faces, one very friendly, one very gloomy...

It is easy to understand that in the dreary middle ages the Aristotelian logic would be very acceptable to the controversial spirit of the schoolmen, which, in the absence of all real knowledge, spent its energy upon mere formulas and words, and that it would be eagerly adopted even in its mutilated Arabian form, and presently established as the centre of all knowledge.

Mankind is growing out of religion as out of its childhood clothes.- On Religion

Religions are like glow-worms: they need darkness in order to shine. A certain degree of general ignorance is the condition for the existence of any religion.

Religions are the children of ignorance, and they do not long survive their mother.- On Religion

In its death throes, we see religion clinging to morality, whose mother it would like to pretend to be. In vain! – genuine morality is dependent on no religion, although religion sanctions and thereby sustains it.

Throughout the entire Christian era theism has lain like an incubus on all intellectual, especially philosophical endeavor and has prevented or stunted all progress; and when anyone has possessed the rare elasticity of mind which alone can slip free of these fetters, his writings have been burned and sometimes their author with them, as happened to Bruno and Vanini.

A man cannot serve two masters: so it is either reason or the scriptures.- On Religion