The Netherlands became a post religious society in a single generation. It wasn't education, it wasn't ideology. It was materialism. Capitalism responded to rising wages with entertainment and leisure. Sunday became fun day.
They also promulgated plenty of verifiably false information. The popular misconception that pre-Columbian Europeans thought the world to be flat is attributable to these two. They also spread ahistorical claims that the Catholic Church was broadly opposed to science and forbade human dissection.
White was Cornell's first president, and his book, "A History of the Warfare of Science with Theology in Christendom," was influential among the public and academics. Much of it had been serialized earlier as articles in Popular Science Monthly, now the magazine known as Popular Science.
In addition to popularizing the idea of an inherent conflict between faith and science, Draper had theories on the causes of the American Civil War. He attributed it to climate and its influence on the personalities of the people who reside in those climates. He proposed building more north-south railroads as a preventative measure.
What seems more likely (to me) is that due to advancements in transport and information technology people increasingly learned about different world religions, and did the math.
If there is only one God, how can other people worship a different one who is also the only one?
Also, religion started to fade during The Enlightenment, and things took a little while to completely crumble.
Apart from this diversion, I do think people heard about other gods alright, but communities used to be quite isolated. It has been possible to collectively ignore other religions for centuries. Technological developments in transport and information distribution, such as the printing press, may have made it increasingly harder to keep a lid on the group think. Seems like a reasonable argument, no?