C.H. Spurgeons's The Treasury of David
Verse 3. Let the house of Aaron now say, that his mercy endureth for ever. The sons of Aaron were specially set apart to come nearest to God, and it was only because of his mercy that they were enabled to live in the presence of the thrice holy Jehovah, who is a consuming fire. Every time the morning and evening lamb was sacrificed, the priests saw the continual mercy of the Lord, and in all the holy vessels of the sanctuary, and all its services from hour to hour, they had renewed witness of the goodness of the Most High. When the high priest went in unto the holy place and came forth accepted, he might, above all men, sing of the eternal mercy. If this Psalm refers to David, the priests had special reason for thankfulness on his coming to the throne, for Saul had made a great slaughter among them, and had at various times interfered with their sacred office. A man had now come to the throne who for their Master's sake would esteem them, give them their dues, and preserve them safe from all harm. Our Lord Jesus, having made all his people priests unto God, may well call upon them in that capacity to magnify the everlasting mercy of the Most High. Can any one of the royal priesthood be silent?