Orthodoxy

Orthodoxy refers to any religious system which has doctrines that must be believed on the basis of the authority of some institution. In this instance, Chesterton is referring to the orthodoxy as put forward by the Catholic Church on the basis of the authority it argues comes from Jesus Christ.

Evolution / Darwinism

The theory of evolution was a popular theory in the time Chesterton was writing (early 20th century). Fundamentally, it was the belief that modern organisms evolved from older, "lower" forms of life. Many took this theory and extended it beyond biology, however, and saw society as constantly evolving and improving.

Progress

Progress is the movement of society towards some fixed vision of what is good. In Chesterton's time, there is much confusion over what this means. Some equate progress simply with the march of time—that is, whatever changes happen are progress, whether they...