Why do the people Les opposes refuse to accept a literal, physical, political, earthly kingdom?
They stand rigidly against the concept of an earthly kingdom because they have spiritualized the Scriptures, treating everything pertaining to the Kingdom as spiritual. Consequently, they refuse to accept the Bible's teaching that the Kingdom is literal, physical, political, and earthly.
Les explains that those who oppose the earthly kingdom have spiritualized the Scriptures, making everything about the Kingdom spiritual. They therefore refuse to accept that the Bible teaches a literal, physical, yes political, earthly kingdom. Les contrasts this with the coming kingdom that includes the animal kingdom, families, tremendous reproduction, and believing parents. He also mentions reading the opposition—the Book of Mormon, the Koran, Armstrong's writings, and Jehovah's Witnesses material—to know where they are coming from.