17 See those inner things commended to us in the psalm: “But the sons of men will put their trust in the covering of Your wings.” See what it is to enter within; see what it is to flee for refuge to His protection; see what it is to run even under the Father's lash, for He scourges every son whom He receives. “But the sons of men shall put their trust under the cover of Your wings.” What is within? “They shall be filled with the plenteousness of Your house,” when You shall have sent them within, entering into the joy of their Lord; “they shall be filled with the plenteousness of Your house; and You shall give them to drink of the stream of Your pleasure.
For with You is the fountain of life.” Not away without You, but within with You, is the fountain of life. “And in Your light we shall see light. Show Your mercy upon them that know You, and Your righteousness to them that are of upright heart.” They who follow the will of their Lord, not seeking their own, but the things of the Lord Jesus Christ, they are the upright in heart, their feet shall not be moved. For “God is good to Israel, to the upright in heart. But, as for me, says he, my feet were almost moved.”
Why? “Because I was jealous at sinners, looking at the peace of sinners.” To whom is God good then, unless to the upright in heart? For God was displeasing to me when my heart was crooked. Why displeasing? Because He gave happiness to the wicked, and therefore my feet tottered, as if I had served God in vain. For this reason, then, my feet were almost moved, because I was not upright of heart. What then is upright in heart? Following the will of God. One man is prosperous, another man toils; the one lives wickedly and yet is prosperous, the other lives rightly and is distressed.
Let not him that lives rightly and is in distress be angry; he has within what the prosperous man has not: let him therefore not be saddened, nor vex himself, nor faint. That prosperous man has gold in his own chest; this other has God in his conscience. Compare now gold and God, chest and conscience. The former has that which perishes, and has it where it will perish; the latter has God, who cannot perish, and has Him there whence He cannot be taken away: only if he is upright in heart; for then He enters within and goes not out.
For that reason, what said he? “For with You is the fountain of life:” not with us. We must therefore enter within, that we may live; we must not be, as it were, content to perish, nor willing to be satisfied of our own, to be dried up, but we must put our mouth to the very fountain, where the water fails not. Because Adam wished to live by his own counsel, he, too, fell through him who had fallen before through pride, who invited him to drink of the cup of his own pride. Wherefore, because “with You is the fountain of life, and in Your light we shall see light,” let us drink within, let us see within.
Why was there a going out thence? Hear why: “Let not the foot of pride come to me.” Therefore he, to whom the foot of pride came, went out. Show that therefore he went out. “And let not the hands of sinners move me;” because of the foot of pride. Why do you say this? “They are fallen, all they that work iniquity.” Where are they fallen? In their very pride. “They were driven out, and they could not stand.” If, then, pride drove them out who were not able to stand, humility sends them in who can stand for ever.
For this reason, moreover, he who said, “The bones that were brought low shall rejoice,” said before, “You shall give joy and gladness to my hearing.” What does he mean by, “to my hearing”? By hearing You I am happy; because of Your voice I am happy; by drinking within I am happy. Therefore do I not fall; therefore “the bones that were brought low will rejoice;” therefore “the friend of the Bridegroom stands and hears Him;” therefore he stands, because he hears. He drinks of the fountain within, therefore he stands. They who willed not to drink of the fountain within, “there are they fallen: they were driven, they were not able to stand.”
Source: Tractates on the Gospel of John (New Advent)