8 Inasmuch, then, as the Spirit of God pointed out by the prophets things to come, forming and adapting us beforehand for the purpose of our being made subject to God, but it was still a future thing that man, through the good pleasure of the Holy Spirit, should see [God], it necessarily behooved those through whose instrumentality future things were announced, to see God, whom they intimated as to be seen by men; in order that God, and the Son of God, and the Son, and the Father, should not only be prophetically announced, but that He should also be seen by all His members who are sanctified and instructed in the things of God, that man might be disciplined beforehand and previously exercised for a reception into that glory which shall afterwards be revealed in those who love God.
For the prophets used not to prophesy in word alone, but in visions also, and in their mode of life, and in the actions which they performed, according to the suggestions of the Spirit. After this invisible manner, therefore, did they see God, as also Esaias says, “I have seen with my eyes the King, the Lord of hosts,” pointing out that man should behold God with his eyes, and hear His voice. In this manner, therefore, did they also see the Son of God as a man conversant with men, while they prophesied what was to happen, saying that He who was not come as yet was present proclaiming also the impassible as subject to suffering, and declaring that He who was then in heaven had descended into the dust of death. Moreover, [with regard to] the other arrangements concerning the summing up that He should make, some of these they beheld through visions, others they proclaimed by word, while others they indicated typically by means of [outward] action, seeing visibly those things which were to be seen; heralding by word of mouth those which should be heard; and performing by actual operation what should take place by action; but [at the same time] announcing all prophetically. Wherefore also Moses declared that God was indeed a consuming fire (igneum) to the people that transgressed the law, and threatened that God would bring upon them a day of fire; but to those who had the fear of God he said, “The Lord God is merciful and gracious, and long-suffering, and of great commiseration, and true, and keeps justice and mercy for thousands, forgiving unrighteousness, and transgressions, and sins.”
Source: Adversus haereses (New Advent)