For innumerable evils have compassed me about - Have surrounded me, or have beset me on every side. The evils here referred to, understood as being those which came upon the Messiah, were sorrows that came upon him in consequence of his undertaking to do what could not be done by sacrifices and offerings Psalm 40:6; that is, his undertaking to save men by his own "obedience unto death." The time referred to here, I apprehend, is that when the full effects of his having assumed the sins of the world to make expiation for them came upon him; when he was about to endure the agonies of Gethsemane and Calvary.

Mine iniquities have taken hold upon me - On this passage, as constituting one of the main objections, and the strongest objection, to the application of the psalm to the Messiah, and on the way in which such objection may be met, see introduction to this psalm (3b).

So that I am not able to look up - This is not the exact idea of the Hebrew word. That is simply, I am not able to see; and it refers to the dimness or failure of sight caused by distress, weakness, or old age. 1 Samuel 3:2; 1 Samuel 4:15; 1 Kings 14:4; compare Psalm 6:7. The idea here is, not that he was unable to look up, but that the calamities which came upon him were so heavy and severe as to make his sight dim, or to deprive him of vision. Either by weeping, or by the mere pressure of suffering, he was so affected as almost to be deprived of the power of seeing.

They are more than the hairs of mine head - That is, the sorrows that come upon me in connection with sin. The idea is that they were innumerable - the hairs of the head, or the sands on the seashore; being employed in the Scriptures to denote what cannot be numbered. See Psalm 69:4. Compare Genesis 22:17; Genesis 32:12; Joshua 11:4; 2 Samuel 17:11.

Therefore my heart faileth me - Margin, as in Hebrew: "forsaketh." The idea is that he sank under these sufferings; he could not sustain them.

Psalm 40:12 Parallel Commentaries

"Lo, I Come": Exposition
"Wherefore when he cometh into the world, he saith, Sacrifice and offering thou wouldest not, but a body hast thou prepared me: in burnt offerings and sacrifices for sin thou hast had no pleasure. Then said I, Lo, I come in (the volume of the book it is written of me,) to do thy will, O God." WE HAVE, in the use made of the passage by the inspired apostle, sufficient authority for applying the quotation from the fortieth psalm to our divine Lord and Savior Jesus Christ. With such a commentary, we …
Charles Haddon Spurgeon—Spurgeon's Sermons Volume 37: 1891

Eligius, Bishop of Noyon.
THE life of this pious bishop is so much the more worthy our consideration, on account of his having passed many years in the position of an ordinary citizen, before he entered on the clerical office; because his life may thus afford us a picture of the pious citizens of his time. Eligius was born at Chatelàt, a mile from Limoges, A. D. 588. His family had been Christian for many generations, and he received a pious education, [8] the result of which extended throughout his life. In his youth, …
Augustus Neander—Light in the Dark Places

The Lamb of God, the Great Atonement
Behold the Lamb of God, which taketh away the sin of the world! G reat and marvellous are the works of the LORD God almighty! We live in the midst of them, and the little impression they make upon us, sufficiently proves our depravity. He is great in the very smallest; and there is not a plant, flower, or insect, but bears the signature of infinite wisdom and power. How sensibly then should we be affected by the consideration of the Whole , if sin had not blinded our understandings, and hardened …
John Newton—Messiah Vol. 1