What metaphors are used in The Everlasting Man by G. K. Chesterton?

In Part II, Chapter IV, Chesterton explains that when Christ founded his Church, he described the authority he gave to his Apostles with the metaphor of a key. He also notes the way in which this metaphor is actually incredibly fitting for the Christian faith. Like a key, the Christian faith promised to unlock the chains that bound mankind to sin. Further, like a key, the Christian faith had a very specific shape which, if it was lost, would compromise the whole.