But purity of heart is to will one thing. It is this thesis…which we have linked to the apostolic words: “Draw nigh to God and He will draw nigh to you, cleanse your hands, ye sinners, and purify your hearts, ye double-minded!” For commitment to the Good is a whole-souled decision, and a person cannot [with words]…lay hold of God while his or her heart is far away. No, for since God is spirit and truth, we can only draw near to Him by sincerity, by willing to ‘be holy, as He is holy’: by purity of heart.
Purity of heart: it is a figure of speech that compares the heart to the sea, and why not just to this? Simply for the reason that the depth of the sea determines its purity, and its purity determines its transparency... When [the sea] is deeply and transparently pure, then it is all of one consistency, no matter how long one looks at it… On this account we compare the heart with the sea, because the purity of the sea lies in its depth and transparency… And today, if you should see it…you would be drawn upwards by contemplating the purity of the sea. If you saw it every day, then you would declare that it is forever sure – like the heart of the human who wills the one thing!
As the sea, when it lies calm and deeply transparent, yearns for heaven, so may the pure heart, when it is calm and deeply transparent, yearn for the Good. As the sea is made pure by yearning for heaven alone; so may the heart become pure by yearning only for the Good. As the sea mirrors the elevation of heaven in its pure depths, so may the heart…mirror the divine elevation of the Good in its pure depths…
Soren Kierkegaard, Purity of Heart Is to Will One Thing, translated from the Danish by Douglas V. Steere (New York: Harper & Brothers, 1938), 158-159.