1 Among all the Songs entitled the Song of degrees, this Psalm has a further addition in the title, that it is “Solomon's.” For thus it is entitled, “A Song of degrees of Solomon.” It has therefore aroused our attention, and caused us to enquire the reason of this addition, “of Solomon.” For it is needless to repeat explanations of the other words, Song of degrees....Solomon was in his time David's son, a great man, through whom many holy precepts and healthful admonitions and divine mysteries have been wrought by the Holy Spirit in the Scriptures.
Solomon himself was a lover of women, and was rejected by God: and this lust was so great a snare unto him, that he was induced by women even to sacrifice to idols, as Scripture witnesses concerning him. But if, by his fall what was delivered through him were blotted out, it would be judged that he had himself delivered these precepts, and not that they were delivered through him. The mercy of God, therefore, and His Spirit, excellently wrought that whatever of good was declared through Solomon, might be attributed unto God; and the man's sin, unto the man.
What marvel that Solomon fell among God's people? Did not Adam fall in Paradise? Did not an angel fall from heaven, and become the devil? We are thereby taught, that no hope must be placed in any among men....The name of Solomon is interpreted to mean peacemaker: now Christ is the True Peacemaker, of whom the Apostle says, “He is our Peace, who has made both one.”...Since, therefore, He is the true Solomon; for that Solomon was the figure of this Peace maker, when he built the temple; that you may not think he who built the house unto God was the true Solomon, Scripture showing unto you another Solomon, thus commences this Psalm: “Except the Lord build the house, their labour is but lost that build it”.
The Lord, therefore, builds the house, the Lord Jesus Christ builds His own house. Many toil in building: but, except He build, “their labour is but lost that build it.” Who are they who toil in building it? All who preach the word of God in the Church, the ministers of God's mysteries. We are all running, we are all toiling, we are all building now; and before us others have run, toiled, and built: but “except the Lord build, their labour is but lost.” Thus the Apostles seeing some fall bewailed these men, in that they had laboured in vain for them. We, therefore, speak without, He builds within.
We can observe with what attention ye hear us; He alone who knows your thoughts, knows what ye think. He Himself builds, He Himself admonishes, He Himself opens the understanding, He Himself kindles your understanding unto faith; nevertheless, we also toil like workmen; but, “except the Lord build,” etc.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)