9 “Moses and Aaron among His priests, and Samuel among such as call upon His Name: these called upon the Lord, and He heard them”. “He spoke unto them out of the cloudy pillar”....Of Moses it is not there stated that he was a priest. But if he was not this, what was he? Could he be anything greater than a priest? This Psalm declares that he also was himself a priest: “Moses and Aaron among His priests.” They therefore were the Lord's priests. Samuel is read of later in the Book of Kings: this Samuel is in David's times; for he anointed the holy David.
Samuel from his infancy grew up in the temple....He mentions these: and by these desires us to understand all the saints. Yet why has he here named those? Because we said that we ought here to understand Christ. Attend, holy brethren. He said above, “O magnify the Lord our God: and fall down before His footstool, for He is holy:” praising some one, that is, our Lord Jesus Christ; whose footstool is to be worshipped, because He assumed flesh, in which He was to appear before the human race; and wishing to show unto us that the ancient fathers also had preached of Him, because our Lord Jesus Christ is Himself the True Priest, he mentioned these, because God spoke unto them out of the cloudy pillar.
What means, “out of the cloudy pillar”? He was speaking figuratively. For if He spoke in some cloud, those obscure words predicted some one unknown, yet to be manifest. This unknown one is no longer unknown; for He is known by us, our Lord Jesus Christ....He who first spoke out of the cloudy pillar, has in Person spoken unto us in His footstool; that is, on earth, when He had assumed the flesh, for which reason we worship His footstool, for He is holy. He Himself used to speak out of the cloud, which was not then understood: He has spoken in His own footstool, and the words of His cloud have been understood.
“They kept His testimonies, and the law that He gave them.”...“You heard them,” he says, “O Lord our God: You were forgiving to them, O God”. God is not said to be forgiving toward anything but sins: when He pardons sins, then He forgives. And what had He in them to punish, so that He was forgiving in pardoning them? He was forgiving in pardoning their sins, He was also forgiving in punishing them. For what follows? “And punished all their own affections.” Even in punishing them You were forgiving toward them: for not in remitting, but also in punishing their sins, have You been forgiving.
Consider, my brethren, what he has taught us here: attend. God is angry with him whom, when he sins, He scourges not: for unto him to whom He is truly forgiving, He not only remits sins, that they may not injure him in a future life; but also chastens him, that he delight not in continual sin.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)