Except the Lord build the house - Or rather, “a house.” The word “house” may refer either to an ordinary dwelling; to the temple, as a place of worship; or to a family, with reference to its success and prosperity, as the word house is often used now. The statement is universal, and is designed to indicate a universal dependence on God in human undertakings, though it is not improbable that there may have been an allusion, when the psalm was composed, to some building which was contemplated or commenced. If the psalm was a composition of David or Solomon, the allusion way have been to the temple about to be erected. The language, however, is so general as to be applicable to any enterprise of that kind.
They labor in vain that build it - literally, “In vain toil its builders in it.” The idea is, that they are entirely dependent on God. No matter what their skill, their strength, their industry may be - all will be in vain unless God shall assist them. They are dependent on Him for life, for health, for strength, for practical wisdom, for a disposition to continue their work, and for success in it. Their work might be destroyed by fire, by a tempest, by an earthquake, or by an irruption of enemies; and for the result, therefore, they are entirely dependent on God.
Except the Lord keep the city - The same idea of dependence is here repeated in another form. The preservation of a city depends wholly on God, whatever care or precaution may be used.
The watchman waketh but in vain - literally, “In vain waketh the keeper.” The word rendered waketh means to be sleepless; and then, to watch. The allusion is to the watch or guard appointed to keep a city, and the idea is, that, whatever may be the diligence, the care, the fidelity of one thus appointed to guard a city, its safe-keeping must depend on God alone. Fires may break out in spite of the watchmen; a tempest may sweep over it; bands of armed people may assail it; or the pestilence may suddenly come into it, and spread desolation through its dwellings. There may have been an allusion in this to some immediate arrangement for guarding Jerusalem when the psalm was composed; but the remark is so general that it is not necessary to confine it to that. It is universally true that, after all the care for their own preservation which people can employ, their safety depends wholly on God.