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.
The Haeckel monists were very influential in Germany. The monist league eventually became an official organ of the Nazi party. The american and british monists went a different route. The Rationalist Press Association was full of monists. They disseminated Haeckel's books in English. In America, Paul Carus formed The Monist journal. It is a mixture of articles by famous scientists and philosophers together with Paul Carus's anti-Christian tripe sandwiched in between.
There are systematic efforts to close minds and render certain topics, and words, forbidden
Agnosticism is a forbidden word among the Monists. Many things are forbidden.
I was thinking how I can give you a short description of monism as I see it from reading The Monist. You are probably familiar with "pagan Christ" theories, viz, Frazer's Golden Bough, Kersey Graves, Acharya S, Gany & Freke's The Jesus Mysteries, Dan Brown, and so on. You will note the peculiar form of reasoning that all of these writers use. The way they patch together explanations of unrelated things that don't make sense, and so on. Hard to describe, but if you've read them then you know what I mean. Take that peculiar form of reasoning about religion and history, and apply it to biology, physics, philosophy etc. You get Monism.
And not only that, Monists are the ones who popularized all those "pagan origins of Christianity" theories. About half of the Monist is like that. And many Rationalist Press books are like that. Indeed Monists, having a fascination for pagan or nature-religion symbolism, are obsessed with such theories. Reminiscent of Ostara and those weird pre-Nazi pagan revival magazines in Germany.
One of the sources I found mentioned the monism of Hegel.
You can probably consider Hegel a precursor to monism (and to british evolutionary philosophy.) Oken, the old German biologist, wrote like a monist, before Haeckel. Nietzche is big with monists. They seem to love him. Monism has a number of idols which are baptized as monists: Goethe, Spinoza, Bruno, etc.
Einstein was a Monist.