This is a rough pencil sketch for an illustration of G.K Chesterton's monumental book Orthodoxy. It shows many prominent atheist and agnostic thinkers of the late 19th and early 20th century: Nietzche, Huxley, Spencer, Bradlaugh and Shaw, all sitting at a table with their books piled in the center of their table. Chesterton said it was actually their writings, and all the contradictions he found in them, that led him to Christianity.
In the illustration Chesterton has left their table and has found the "key that fits", the key of truth that he is using to unlock the door into the Church. The men at the table show frustration and disbelief at Gilbert's decision.