This psalm purports to be a psalm of David, and there is no reason to doubt that it is properly attributed to him. See introduction to Psalm 138:1-8. At what time it was composed is, however, unknown. It contains reflections which might have occurred at any period of his life; yet it would seem most probable that it was not written in his early years, but that it is a record of his most mature thoughts on a great and very important subject.
The psalm relates to the omnipresence of God, and contains such reflections as would occur to one meditating on that attribute of the Deity. It is the most distinct and full statement of that doctrine which is to be found in the Hebrew Scriptures, and the doctrine is presented in language which has never been surpassed for sublimity and beauty. The leading idea in the psalm seems to be that of comfort from the fact that God is everywhere; that he knows all that pertains to us; that we can never be hidden from his view; that he has known us from the beginning; that as he fashioned and formed us - making us what we are - he knows all our necessities, and can supply them. The psalm consists of three parts:
I. A celebration of the Omniscience and Omnipresence of God, as a ground of confidence and hope, Ps. 139:1-18.
(a) The fact that he knows all that there is in the heart, Psalm 139:1-6.
(b) The fact that he is everywhere present, Psalm 139:7-12.
(c) The fact that all in our past life has been known to God; that he has created us, and that his eye has been upon us from the beginning of our existence, Psalm 139:13-16.
(d) The fact that his thoughts toward us are precious, and numberless as the sand, Psalm 139:17-18.
II. The feelings of the psalmist in relation to the acts of the wicked as a proof that he loved God, Psalm 139:19-22. These reflections seem to have sprung from his contemplation of the divine character and perfections, as leading him to hate all that was opposed to a Being so pure, so benevolent, so holy. On looking into his own heart, in view of what God was, he was conscious that he had no sympathy with the enemies of God as such; that such was his love for the character of God, and such his confidence in him, that he could have nothing in common with them in their feelings toward God, but wished to be dissociated from them forever.
III. The expression of a desire that, as God saw all the recesses of the human soul, he would search his heart, and would detect any evil he might see there, and deliver him from the evil, and lead him in the way which conducted to life eternal, Psalm 139:23-24. Anyone may feel, and must feel, that after all which he knows of himself - after all the effort which he makes to ascertain what is within his heart - there are depths there which his eye cannot penetrate, and that there may be sins of thought and feeling there which he has not detected; but it is only from the consciousness of sincerity, and a true desire to honor God, that one can pray that God would search him, and that he would detect and bring out every form of sin which he may see concealed and lurking in the soul He who can sincerely offer this prayer is a pious man.

A fine account of the omniscience of God, Psalm 139:1-6; of his omnipresence, Psalm 139:7-12; of his power and providence Psalm 139:13-16. The excellence of his purposes, Psalm 139:17, Psalm 139:18. His opposition to the wicked, Psalm 139:19, Psalm 139:20; with whom the godly can have no fellowship, Psalm 139:21, Psalm 139:22.
The title of this Psalm in the Hebrew is, To the chief Musician, or, To the Conqueror, A Psalm of David. The Versions in general follow the Hebrew. And yet, notwithstanding these testimonies, there appears internal evidence that the Psalm was not written by David, but during or after the time of the captivity, as there are several Chaldaisms in it. See Psalm 139:2, Psalm 139:3, Psalm 139:7, Psalm 139:9, Psalm 139:19, Psalm 139:20, collated with Daniel 2:29, Daniel 2:30; Daniel 4:16; Daniel 7:28; some of these shall be noticed in their proper places.
As to the author, he is unknown; for it does not appear to have been the work of David. The composition is worthy of him, but the language appears to be lower than his time.
Concerning the occasion, there are many conjectures which I need not repeat, because I believe them unfounded. It is most probable that it was written on no particular occasion, but is a moral lesson on the wisdom, presence, providence, and justice of God, without any reference to any circumstance in the life of David, or in the history of the Jews.
The Psalm is very sublime; the sentiments are grand, the style in general highly elevated, and the images various and impressive. The first part especially, that contains so fine a description of the wisdom and knowledge of God, is inimitable.
Bishop Horsley's account of this Psalm is as follows: -
"In the first twelve verses of this Psalm the author celebrates God's perfect knowledge of man's thoughts and actions; and the reason of this wonderful knowledge, viz., that God is the Maker of man. Hence the psalmist proceeds, in the four following verses, Psalm 139:13-16, to magnify God as ordaining and superintending the formation of his body in the womb. In the 17th and 18th (Psalm 139:17, Psalm 139:18) he acknowledges God's providential care of him in every moment of his life; and in the remainder of the Psalm implores God's aid against impious and cruel enemies, professing his own attachment to God's service, that is, to the true religion, and appealing to the Searcher of hearts himself for the truth of his professions."
The composition, for the purity and justness of religious sentiment, and for the force and beauty of the images, is certainly in the very first and best style. And yet the frequent Chaldaisms of the diction argue no very high antiquity.

INTRODUCTION TO PSALM 139
To the chief Musician, A Psalm of David. This psalm was written by David, when he lay under the reproach and calumnies of men, who laid false things to his charge; things he was not conscious of either in the time of Saul's persecution of him, or when his son Absalom rebelled against him: and herein he appeals to the heart searching and rein trying God for his innocence; and, when settled on his throne, delivered it to the master of music, to make use of it on proper occasions. According to the Syriac title of the psalm, the occasion of it was Shimei, the son of Gera, reproaching and cursing him as a bloody man, 2-Samuel 16:5. Theodoret takes it to be a prophecy of Josiah, and supposes that he is represented as speaking throughout the psalm. Aben Ezra observes, that this is the most glorious and excellent psalm in all the book: a very excellent one it is: but whether the most excellent, it is hard to say. It treats of some of the most glorious of the divine perfections; omniscience, omnipresence, and omnipotence. Arama says, the argument of it is God's particular knowledge of men, and his providence over their affairs.

(Psalm 139:1-6) God knows all things.
(Psalm 139:7-16) He is every where present.
(Psalm 139:17-24) The psalmist's hatred to sin, and desire to be led aright.

Adoration of the Omniscient and Omnipresent One
In this Aramaizing Psalm what the preceding Psalm says in Psalm 139:6 comes to be carried into effect, viz.: for Jahve is exalted and He seeth the lowly, and the proud He knoweth from afar. This Psalm has manifold points of contact with its predecessor. From a theological point of view it is one of the most instructive of the Psalm, and both as regards its contents and poetic character in every way worthy of David. But it is only inscribed לדוד because it is composed after the Davidic model, and is a counterpart to such Psalm as Psalm 19:1-14 and to other Davidic didactic Psalm. For the addition למנצח neither proves its ancient Davidic origin, nor in a general way its origin in the period prior to the Exile, as Ps 74 for example shows, which was at any rate not composed prior to the time of the Chaldaean catastrophe.
The Psalm falls into three parts: Psalm 139:1, Psalm 139:13, Psalm 139:19; the strophic arrangement is not clear. The first part celebrates the Omniscient and Omnipresent One. The poet knows that he is surrounded on all sides by God's knowledge and His presence; His Spirit is everywhere and cannot be avoided; and His countenance is turned in every direction and inevitably, in wrath or in love. In the second part the poet continues this celebration with reference to the origin of man; and in the third part he turns in profound vexation of spirit towards the enemies of such a God, and supplicates for himself His proving and guidance. In Psalm 139:1 and Psalm 139:4 God is called Jahve, in Psalm 139:17 El, in Psalm 139:19 Eloha, in Psalm 139:21 again Jahve, and in Psalm 139:23 again El. Strongly as this Psalm is marked by the depth and pristine freshness of its ideas and feeling, the form of its language is still such as is without precedent in the Davidic age. To all appearance it is the Aramaeo-Hebrew idiom of the post-exilic period pressed into the service of poetry. The Psalm apparently belongs to those Psalm which, in connection with a thoroughly classical character of form, bear marks of the influence which the Aramaic language of the Babylonian kingdom exerted over the exiles. This influence affected the popular dialect in the first instance, but the written language also did not escape it, as the Books of Daniel and Ezra show; and even the poetry of the Psalm is not without traces of this retrograde movement of the language of Israel towards the language of the patriarchal ancestral house. In the Cod. Alex. Ζαχαρίου is added to the τῷ Δαυίδ ψαλμός, and by a second hand ἐν τῇ διασπορᾷ, which Origen also met with "in some copies."