When Christian and Hopeful are taken to the Doubting Castle by Giant Despair, Christian remembers the key in his bosom.  But what specifically is this key? 

It is a synonym for the truth of Christian’s book or Bible embedded in his bosom or heart, though recently neglected. More particularly, it is the encouragement of Christ to those given to him by the Father. When we find ourselves to be "down" we need to remember to look "up."  And who better to look to than the source of truth. Christian realized he had neglected to be in the word. The way to change ones heart is to first change the mind. How do you change the mind? You must focus on the truth and we find that truth in God's word.