A major theme of the fifth chapter, the act of martyrdom is that by which an individual is killed on account of their religious beliefs. An English loan word from ancient Greek, the word comes from the verb “to witness/give testimony.” Thus, a martyr is one who gives witness to the truth of their convictions by their refusal to abandon them even in the face of certain death. As Chesterton outlines in the book, the act of Christian martyrdom is one in which the individual is willing to sacrifice themselves for something that they hold even more dear and precious than their own life.

Though the word is given a number of definitions depending on the context, as Chesterton himself acknowledges, materialism here is typically used regarding the philosophy that holds physical, material things as the only things that exist in the universe. Materialism, by definition, rules out the existence of anything that is not composed of matter or that cannot be detected and studied by the physical sciences (such as biology, chemistry, or physics).