15 Return then with me to the face of the heart, and make it ready. That to which God speaks is within. The ears, and eyes, and all the rest of the visible members, are either the dwelling place or the instrument of some thing within. It is the inner man where Christ does dwell, now by faith, and hereafter He will dwell in it, by the presence of His Divinity, when we shall have known “what is the length, and breadth, and depth, and height; when we shall have known also the love of Christ that surpasses knowledge, that we may be filled with all the fullness of God.” Now then if you would enter into the meaning of these words, summon all your powers to comprehend the breadth, and length, and height, and depth.
Wander not in the imagination of the thoughts through the spaces of the world, and the yet comprehensible extent of this so vast a body. Look for what I am speaking of in your own self. The “breadth” is in good works; the “length” is in long-suffering and perseverance in well-doing; the “height” is in the expectation of rewards above, for which height's sake you are bidden “to lift up your heart.” Do well, and persevere in well-doing, because of God's reward. Esteem earthly things as nothing, lest, when this earth shall be smitten with any scourge of that wise One, you say that you have worshipped God in vain, hast done good works in vain, hast persevered in good works in vain.
For by doing good works you had as it were the “breadth,” by persevering in them you had as it were the “length;” but by seeking earthly things you have not had the “height.” Now observe the “depth;” it is the grace of God in the secret dispensation of His will. “For who has known the mind of the Lord? Or who has been His counsellor?” and, “Your judgments are as a great depth.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)