Ps 42:4

“Now then that we have been led on even to "the Tabernacle," and are placed on "His holy Hill," what hope do we carry with us? "Then will I go in unto the Altar of God." For there is a certain invisible Altar on high, which the unrighteous man approaches not. To that Altar he alone draws nigh, who draws nigh to this one without cause to fear. There he shall find his Life, who in this one "separates his cause." "And I will go in unto the Altar of God." From His holy Hill, and from His Tabernacle, from His Holy Church, I will go in unto the Altar of God on High. What manner of Sacrifice is there? He himself who goeth in is taken for a burnt-offering. "I will go in unto the Altar of God." What is the meaning of what he says, "The Altar of my God"? "Unto God, who makes glad my youth." Youth signifies newness: just as if he said, "Unto God, who makes glad my newness." It is He who makes glad my newness, who hath filled my old estate with mourning. For now "I go mourning" in oldness, then shall "I stand," exulting in newness! "Yea, upon the harp will I praise Thee, O God my God." What is the meaning of "praising on the harp," and praising on the psaltery? For he does not always do so with the harp, nor always with the psaltery. These two instruments of the musicians have each a distinct meaning of their own, worthy of our consideration and notice. They are both borne in the hands, and played by the touch; and they stand for certain bodily works of ours. Both are good, if one knows how to play the psaltery, or to play the harp. But since the psaltery is that instrument which has the shell (i.e. that drum, that hollow piece of wood, by straining on which the chords resound) on the upper part of it, whereas the harp has that same concave sounding-board on the lower part, there is to be a distinction made between our works, when they are "upon the harp," when "on the psaltery:" both however are acceptable to God, and grateful to His ear. When we do anything according to God's Commandments, obeying His commands and hearkening to Him, that we may fulfil His injunctions, when we are active and not passive, it is the psaltery that is playing. For so also do the Angels: for they have nothing to suffer. But when we suffer anything of tribulation, of trials, of offences on this earth (as we suffer only from the inferior part of ourselves; i.e. from the fact that we are mortal, that we owe somewhat of tribulation to our original cause, and also from the fact of our suffering much from those who are not "above"); this is "the harp." For there rises a sweet strain from that part of us which is "below:" we "suffer," and we strike the psaltery, or shall I rather say we sing and we strike the harp. ...”

“Youth. St. Jerome, “the God of my joy and exultation.” (Haydock) — Syriac and Arabic agree with us, and Gil means, (Calmet) a young man, in Arabic and Hebrew. (Hammond) — People in youth, shew for the effects of joy. (Berthier) — Accompanied with light, and a pure intention, we may offer sacrifice to God, who changeth our corruptions into newness of life. (Worthington) — The Levites might sing near the altar, but could not offer victims. (Calmet)”

“The poet, in anticipation, revels in the thought of that which he has prayed for, and calls upon his timorous soul to hope confidently for it. The cohortatives in Psa 43:4 are, as in Ps 39:14 and frequently, an apodosis to the petition. The poet knows no joy like that which proceeds from God, and the joy which proceeds from Him he accounts as the very highest; hence he calls God אל שׂמחת גּילי, and therefore he knows no higher aim for his longing than again to be where the fountainhead of this exultant joy is (Hos 9:5), and where it flows forth in streams (Psa 36:9). Removed back thither, he will give thanks to Him with the cithern (Beth instrum.). He calls Him אלהים אלהי, an expression which, in the Elohim-Psalms, is equivalent to יהוה אלהי in the Jahve-Psalms. The hope expressed in Psa 43:4 casts its rays into the prayer in Psa 43:3. In Psa 43:5, the spirit having taken courage in God, holds this picture drawn by hope before the distressed soul, that she may therewith comfort herself. Instead of wthmy, Psa 42:6, the expression here used, as in Ps 42:12, is וּמה־תּהמי. Variations like these are not opposed to a unity of authorship.”