15 “My bone is not hid from You, which You have made in secret”. “His bone,” he says. What the people call ossum, is in Latin called os. This is the word in the Greek. For we might think the word os is here the one which makes in the plural ora, not os (short), which makes ossa. He says then, I have a certain bone (ossum) in secret. For this word let us prefer to use; better is it that scholars find fault with us, than that the people understand us not. “There is then,” says he, a certain bone of mine, within, hidden; You have made within a bone for me in secret, yet is it not hidden from You. In secret have You made it, but have You therefore hidden it from Yourself? This my bone made by You in secret men see not, men know not: You know, who hast made. What “bone” then means he, brethren? Let us seek it, it is “in secret.” But because as Christians we are speaking in the Name of the Lord to Christians, now we find what bone is of this kind. It is a sort of inward strength; for strength and fortitude are understood to be in the bones. There is then a sort of inward strength of the soul, wherein it is not broken. Whatever tortures, whatever tribulations, whatever adversities rage around, that which God has made strong in secret in us, cannot be broken, yields not. For by God is made a certain strength of patience, of which is said in another Psalm, “But my soul shall be subjected to God, for of Him is my patience.”...Wherein do you glory? “In tribulations, knowing that tribulation works patience.” See how that strength is fashioned within in his heart: “because the love of God is shed abroad in our hearts by the Holy Ghost which is given unto us.” So is fashioned and made strong that hidden bone, that it makes us even to glory in tribulations. But to men we seem wretched, because that which we have within is hidden from them. “And my substance is in the lower parts of the earth.” Behold, in flesh is my substance, yet have I a bone within, which You have fashioned, such as to cause me never to yield to any persecutions of this lower region, where still my substance is. For what great matter is it, if an Angel be brave? This is a great matter, if flesh is brave. And whence is flesh brave, whence is an earthen vessel brave, save because in it is made a bone in secret?
16....“Your eyes did see Mine imperfect one, and in Your book shall all be written”, not only the perfect, but also the imperfect. Let not the imperfect fear, only let them advance. Nor yet, because I have said, “let them not fear,” let them love their imperfection, and remain there, where they are found. Let them advance, as far as in them lies. Daily let them add, daily let them approach; yet let them not fall back from the Body of the Lord: that, compacted in one Body and among these members, they may be counted worthy to have that said of them. “By day shall they wander, and none among them.” “The Day” was yet on earth, even our Lord Jesus Christ. Whence He said, “Walk while you have the day.” But “by day shall” His imperfect ones “wander.” They too thought that our Lord Jesus Christ was only man, that He had not within Him the hidden Godhead, that He was not secretly God, but that He was that only which was seen: this they too thought....But what is, “In the day they shall wander”? Shall they perish? Where then is, “In Your book shall all be written”? When then did they “wander in the day”? When they understood not the Lord set upon earth. And what follows? “But to me Your friends are made very honourable, O God”; those very ones, who “wandered in the day, and none was in them,” became Your friends, and were made very honourable to me. That bone was made in them in secret after the resurrection of the Lord, and they suffered for His Name, at whose death they had been amazed. “Mightily strengthened were their chieftainships.” They became Apostles, they became leaders of the Church, they became rams leading their flocks, “mightily strengthened.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)