Interesting that he links both Islam and the “World Religion” to Arianism, which I have always thought to be the case although it is something many people don’t recognize.
Christianity was weakened by Arianism, but not only in a doctrinal sense: it left considerable division among churches, and those in the Middle East (always a fertile source of heresies) were particularly fragmented and isolated from both Rome and Constantinople. I don’t think he could say Christians “willingly” converted, but they were in a position where they were very vulnerable and there was no outside power to defend them.
The same was true in Spain. The Visigothic invaders were Arians, although they were in reality a tiny group of people and their religion had little impact on the majority of the population, who were Catholics. Eventually, one of the Visigothic kings converted to orthodox Catholicism and Arianism was finished in Spain. However, the damage was done: there had been bitter fighting and division between the various Spanish kings, and it was in fact one of these kings who invited the Muslims to come in as mercenary troops to support him in a war against another king. They came in, overran this king’s kingdom and killed him, and then proceeded to sweep through the rest of Spain.
The moral of the story is that bad doctrine weakens not only the actual religious faith of the people, making them more likely to adopt syncretism and abandon Christianity, but that it weakens even the secular society itself. Orthodox Trinitarian doctrine is essential to Christianity, and when the unity of doctrine is broken, the churches are vulnerable.
Just adding to the discussion.
-Theo