Richard C. Carrier said this quote

...the naive forms of Christian moral motivation - bare threats of hell and the bribery of heaven - stunt moral growth by ensuring believers remain emotional children, never achieving the cognitive moral development of adults. Psychologists have established that mature adults are moral not because of bare threats and bribes (that stage of moral development typifies children, not adults), but because they care about the effects their behavior has on themselves and others.

~ Richard C. Carrier

Religious intolerance is an idea that found its earliest expression in the Old Testament, where the Hebrew tribe depicts itself waging a campaign of genocide on the Palestinian peoples to steal their land. They justified this heinous behavior on the grounds that people not chosen by their god were wicked and therefore did not deserve to live or keep their land. In effect, the wholesale slaughter of the Palestinian peoples, eradicating their race with the Jew's own Final Solution, was the direct result of a policy of religious superiority and divine right. Joshua 6-11 tells the sad tale, and one needs only read it and consider the point of view of the Palestinians who were simply defending their wives and children and the homes they had built and the fields they had labored for. The actions of the Hebrews can easily be compared with the American genocide of its native peoples - or even, ironically, the Nazi Holocaust. With the radical advent of Christianity, this self-righteous intolerance was borrowed from the Jews, and a new twist was added. The conversion of infidels by any means possible became the newfound calling card of religious fervor, and this new experiment in human culture spread like wildfire. By its very nature, how could it not have? Islam followed suit, conquering half the world in brutal warfare and, much like its Christian counterpart, it developed a new and convenient survival characteristic: the destruction of all images and practices attributed to other religions. Muslims destroyed millions of statues and paintings in India and Africa, and forced conversion under pain of death (or by more subtle tricks: like taxing only non-Muslims), while the Catholic Church busily burned books along with pagans, shattering statues and defacing or destroying pagan art - or converting it to Christian use. Laws against pagan practices and heretics were in full force throughrout Europe by the sixth century, and as long as those laws were in place it was impossible for anyone to refuse the tenets of Christianity and expect to keep their property or their life. Similar persecution and harassment continues in Islamic countries even to this day, officially and unofficially.

~ Richard C. Carrier

Colonization of the world, more often than not by robbery and warfare, spread Christianity into the Americas and other corners of the earth, just as Islam was spread throughout Asia and Africa. lt is not a coincidence that the two most widespread religions in the world today are the most warlike and intolerant religions in history. Before the rise of Christianity, religious tolerance, including a large degree of religious freedom, was not only custom but in many ways law under the Roman and Persian empires. They conquered for greed and power, rarely for any declared religious reasons, and actually sought to integrate foreign religions into their civilization, rather than seeking to destroy them. People were generally not killed because they practiced a different religion. Indeed, the Christians were persecuted for denying that the popular gods existed — not for following a different religion. In other words, Christians were persecuted for being intolerant.

~ Richard C. Carrier