Who is "Pure in Heart"?

In studying the lesson on which I'll be preaching this Sunday, Matthew 5:1-12 (The Beatitudes), I've been looking at Dietrich Bonhoeffer's chapter on the passage in The Cost of Discipleship. I love what he says about, "Blessed are the pure in heart; for they shall see God":

Who is pure in heart? Only those who have surrendered their hearts completely to Jesus that he may reign in them alone. Only those whose hearts are undefiled by their own evil-and by their own virtues too. The pure in heart have a child-like simplicity like Adam before the fall, innocent alike of good and evil: their hearts are not ruled by their conscience, but by the will of Jesus. 

The old saw about letting your conscience be your guide is foolish nonsense. Unless our consciences are guided by the God revealed in Jesus Christ, we will always drift into self-delusion, harm to ourselves and others, and dishonor to the One Who made us. (I know. Being an imperfect sinner, I "drift" a lot, which is why I repent--that is, turn back to Jesus for forgiveness and new direction and new life every day.)

Martin Luther famously told the Diet of Worms, "My conscience is captive to the Word of God." God grant that I will be just such a captive and so be set free to be an innocent child of God, my true self.