Christianity added new impetus to the expansion of empire. Increasing the arrogance of the imperial project, Christians insisted that the Gospels and the Church were the only valid sources of religious beliefs. Imperialists could claim that they were both civilizing the world and spreading the true religion. By the 5th century, Christianity was thought of as co-extensive with the "Imperium romanum". This meant that to be human, as opposed to being a natural slave, was to be "civilized" and Christian. Historian Anthony Pagden argues, "just as the "civitas"; had now become coterminous with Christianity, so to be human to be, that is, one who was 'civil', and who was able to interpret correctly the law of nature one had now also to be Christian." After the fifteenth century, most Western colonialists rationalized the spread of empire with the belief that they were saving a barbaric and pagan world by spreading Christian civilization.