heathen humanitarians
The words mean what you think they mean, with some nuance. Heathen here means secular, while humanitarian refers to people who help others. It is somewhat pejorative here however, in the sense that Chesterton views them as people who think they know better due to their secular knowledge and are overly willing to interfere with others. He is questioning their knowledge and especially their willingness to interfere in others life because of it.