For thou hast possessed my reins - The word here rendered "possessed" means properly to "set upright," to "erect," and hence, the derivative of the verb is applied to a cane or reed, as being erect. Then the word means to found, to create, Genesis 14:19, Genesis 14:22 - as the heavens and the earth; and then, to get, to gain, to purchase, etc. Here the word seems to be used in its original sense, to make, create, etc. The idea is, not as in our translation, that God "possessed" or "owned" them but that he had "made" them, and that, "therefore," he knew all about them. The word "reins" means literally the "kidneys;" and then, it comes to denote the inward part, the mind, the soul, the seat of the desires, affections, and passions. Jeremiah 11:20. See Psalm 7:9, note; Job 19:27, note. The meaning here is, that God had made him; that the innermost recesses of his being had been constituted as they are by God; and that, "therefore," he must be able to see all that there is in the very depths of the soul, however it may be hidden from the eye of man.

Thou hast covered me in my mother's womb - The word here rendered "cover" means properly to interweave; to weave; to knit together, and the literal translation would be, "Thou hast "woven" me in my mother's womb, meaning that God had put his parts together, as one who weaves cloth, or who makes a basket. So it is rendered by DeWette and by Gesenius (Lexicon). The original word has, however, also the idea of protecting, as in a booth or hut, woven or knit together - to wit, of boughs and branches. The former signification best suits the connection; and then the sense would be, that as God had made him - as he had formed his members, and united them in a bodily frame and form before he was born - he must be able to understand all his thoughts and feelings. As he was not concealed from God before he saw the light, so he could not be anywhere.

Psalm 139:13 Parallel Commentaries

God Omnipresent and Omniscient --Ps. cxxxix.
God Omnipresent and Omniscient--Ps. cxxxix. Searcher of hearts! to Thee are known The inmost secrets of my breast At home, abroad, in crowds, alone, Thou mark'st my rising and my rest, My thoughts far off, through every maze, Source, stream, and issue,--all my ways. How from Thy presence should I go, Or whither from Thy Spirit flee, Since all above, around, below, Exist in Thine immensity? If up to heaven I take my way, I meet Thee in eternal day. If in the grave I make my bed With worms and dust, …
James Montgomery—Sacred Poems and Hymns

The Love of Christ.
THE Patience of Christ was recently the object of our meditation in these pages. Blessed and inexhaustible it is. And now a still greater theme is before our hearts. The Love of Christ. The heart almost shrinks from attempting to write on the matchless, unfathomable love of our blessed and adorable Lord. All the Saints of God who have spoken and written on the Love of Christ have never told out its fulness and vastness, its heights and its depths. "The Love of Christ which passeth knowledge" (Ephesians …
Arno Gaebelein—The Lord of Glory

The Kingdom Undivided
THE POETICAL BOOKS: Psalms Page Song of Solomon Page Proverbs Page THE PSALMS I. The Collection and Divisions: In all probability the book of one hundred and fifty psalms, as it now stands, was compiled by Ezra about 450 B.C. They are divided into five books, each closing with a benediction, evidently added to mark the end of the book. Note the number of psalms in Books 1 and 2. II. The Purposes: 1. They were originally used as songs in the Jewish Temple Worship. …
Frank Nelson Palmer—A Bird's-Eye View of the Bible