Monism is really atheism combined with a certain scientific-sounding nature worship, or just naturalism with different (continental) expressions and idioms. Ernst Haeckel was the big prophet of Monism. He started the Monist League. Many famous guys were members (Ernst Mach, for instance) It was extremely influential. Nothing really explains the 'monist attidude' better than the writings of monists themselves. Here:
The Non-Miraculous History of Creation, Ernst Haeckel
Ernst Haeckel His Life, Works, Career, and Prophecy, T. Wakeman
I never heard of the Monist League before. I don’t doubt their influence.
There are many people who are either monists, or fellow travelers with monism. One need only consider the near hysterical response from some quarters whenever it is suggested that something highly improbable (and meaningful) might be the result of intelligent design.
The word “teleology” probably has the power to set people off as much as the phrase “intelligent design.” Nor is it the word “information” welcome in many discussions of DNA. There are systematic efforts to close minds and render certain topics, and words, forbidden.
One of the sources I found mentioned the monism of Hegel. I’m not certain, but it sounded like his monism was based upon mind/self/(soul) rather than experiences from the five senses, or on matter/energy. That would certainly be a different take than the approach of the Monist league. It might provide the basis for a useful rejoinder, if only as a way to show that there is more than one reasonable way to look at the world.