For starters, The Cost of Discipleship was a fairly difficult read, so it took a while of going back and back to get the meat of what Bonhoeffer was trying to say. Though the over-arching subject is surface level and easy to grasp, it's the smaller points and connection to scripture, which make up the subject, that cause one to heavily focus on.
"What has cost God much cannot be cheap for us."
The opening chapter "Costly Grace", sets up the rest of the book. The above quote really resonated in me; how serious this Jesus stuff really is. God sacrificed His own Son for us...I think sometimes we should be more serious about our faith and choice to follow Jesus as Bonhoeffer stresses so greatly.
In The Cost of Discipleship, Bonhoeffer outlines and goes through what he believes is the way a disciple of Christ should live. The book is broken down into four parts: "Grace and Discipleship", "The Sermon on the Mount", "The Messengers", and "The Church of Jesus Christ and the Life of Discipleship".
In a later chapter Bonhoeffer says that "discipleship means adherence to Christ, and, because Christ is the object of that adherence, it must take the form of discipleship." Bonhoeffer always stresses the fact that discipleship is all about Jesus. That when we give our lives over to Christ, we literally do that...everything we do, from our waking up to the time we go to bed, is for the cause of Christ.
The Cost of Discipleship drives this point home by using the apostles as the example. Bonhoeffer's response to "ye are the salt of the earth": "The word speaks of their whole existence in so far as it is grounded anew in the call of Christ, that same existence which was the burden of the beatitudes. The call of Christ makes those who respond to it the salt of the earth in their total existence."
That is the tone that Bonhoeffer sets in The Cost of Discipleship. To be a disciple of Christ is to make it your sole purpose. Bonhoeffer, through slow reading, drives this home.