Nicene and Post-Nicene Fathers, Vol. II:
On Christian Doctrine: Chapter 10

Chapter 10.—To See God, the Soul Must Be Purified.

10.  Wherefore, since it is our duty fully to enjoy the truth which lives unchangeably, and since the triune God takes counsel in this truth for the things which He has made, the soul must be purified that it may have power to perceive that light, and to rest in it when it is perceived.  And let us look upon this purification as a kind of journey or voyage to our native land.  For it is not by change of place that we can come nearer to Him who is in every place, but by the cultivation of pure desires and virtuous habits.