Psalms 99:6-8

6. Moses and Aaron. The Psalmist magnifies the special grace which God in a very remarkable manner vouchsafed to the seed of Abraham, that thence he chose for himself prophets and priests to be, as it were, mediators between him and the people, to ratify the covenant of salvation. And he mentions three persons who were famous in former times. For Moses was, as it were, a mediator to reconcile the people unto God. Aaron was invested with the same office; and, subsequently, Samuel sustained the same character. There is no doubt, however, that under these three persons he included all the people with whom God had made a covenant. But he mentions the names of those who were the depositaries and guardians of this invaluable treasure. It may appear improper that he should speak of Moses as among the priests, since his sons were only among the common Levites, and that Moses himself, after the giving of the law, never held the office of high priest. But as the Hebrews call כוהנים, chohanim, those who are chief and very eminent personages, such as kings’ sons, there is nothing to prevent the prophet from giving this designation to Moses, as if he had said that he was one of the holy rulers of the Church. Moreover, if we go back to the first original — to the period prior to the publication of the law, it is certain that Moses was then invested with the high priest’s office. The design of the prophet must also be kept in mind, namely, that God not only adopted the seed of Abraham, but set apart some of them to act as mediators, whom he enjoined to call upon his name, in order that his covenant might be the more confirmed. For the invocation of which he speaks must not be understood indiscriminately of every manner of calling upon, but only of that which belongs to the priests, who were chosen by God, as intercessors to appear in his presence in the name of all the people, and to speak on their behalf.

They called upon Jehovah The Psalmist explains more fully what I have just now said, that God from the very first, and with a special reference to his gracious covenant, bestowed great benefits upon the descendants of Abraham — the Jews. And, therefore, as often as they experienced the loving-kindness of God, it behooved them to call to mind his former loving-kindness. The prophet, too, makes particular mention of the visible symbol of the cloudy pillar, by which God designed to testify in all ages that his presence was ever with his people, according as he employed temporal signs, not only for their benefit to whom they were exhibited, but also for the benefit of those who were to succeed them. Not that God always showed a cloudy pillar to his ancient people, but considering that the dullness of men is so great, that they do not perceive the presence of God unless they are put in mind by external signs, the prophet very properly reminds the Jews of this memorable token. And as God had appeared openly in the desert to their fathers, so their posterity might be well assured that he would also be near to them. He adds, that they had kept God’s testimonies, for the purpose of enforcing the duty of like obedience upon succeeding generations.