Man does not see God by his own powers. God is seen by men only when He pleases, only by those He has chosen, only when and how He wills it. God is powerful in all things and He has been seen in times past in a prophetic manner through the Spirit; He has also been seen in an adopted manner through the Son, and He will be seen as Father in the Kingdom of heaven. The Spirit shall indeed prepare man in the Son of God, the Son lead him to the Father, and the Father grant incorruption for eternal life which comes to everyone who looks upon God. For as those who see the light are within the light and share in its brilliancy, even so, those who see God are within God and receive of His splendor. His splendor gives them life and so those who see God are brought to life. It was for this reason that although He is incomprehensible, boundless and invisible, He made Himself visible and comprehensible and within the capacity of those who believe, in order that He might bring to life all those who receive Him and look upon Him through faith. Just as His greatness is past finding out, so is His goodness beyond telling. Granting the vision in His goodness, He then bestows life on all who see Him. It is impossible to live apart from life, and the means of life is found in fellowship with God, and fellowship with God is to know God and to enjoy His goodness. Man, therefore shall see God in order that he may live, being made immortal by the vision and attaining even to God.
From the very beginning the Son of the Father declares Him, since He was with the Father from the beginning. It was He who showed the human race prophetic visions and various gifts, as well as His own ministries and the glory of the Father; all in regular and connected order at the fitting time for the benefit of mankind. For where there is a regular succession there is stability, and where there is stability it is suitable for the time, and where there is suitability there is also utility. This is why the Word became the dispenser of the Father’s grace for the benefit of men, for whose sake He made such great dispensations. He truly revealed God to men, but also presented man to God, all the while preserving the invisibility of the Father so that no man should ever come to despise God, but should always have something to strive forward to. And yet He revealed God to man by man dispensations in case man should fall away from God altogether and so cease to exist. For the glory of God is a man who is alive; and the life of man is the vision of God. If the revelation of God which comes through the creation gives life to all things on earth, then how much more will the revelation of the Father which comes through the Word give life to those who see God. The Spirit of God pointed out things to come by means of the prophets, to form us and adapt us in advance that we should be made subject to God, but it still lay in the future that man through the good-pleasure of the Holy Spirit, should see God. Since the prophets had been the instruments that announced future things, intimating that men would see God, it was fitting that they themselves should see God; not only that God and the Son of God (both Son and Father) should be announced by the prophets, but that He should also be seen by all His members who are sanctified and instructed in divine things. This was so that man might be disciplined and exercised beforehand to prepare for his reception into that glory which will be later revealed in those who love God. The prophets did not only give prophecy in words, but also in visions, and their manner of life, and by the actions which they performed according to the impulse of the Holy Spirit. It was in this invisible manner then that they saw God, as Isaiah tells us: “I have seen with my eyes the King, the Lord of Hosts” (Is. 6:5). He points out that man should look on God with his eyes, and also hear His voice. It was in this same way that they also saw the Son of God, as a man conversant with men, even while they prophesied what was to happen. For they told us that the one who had not yet come was present, and proclaimed that the impassible was subject to suffering, and declared that He who was then in heaven, had descended into the dust of death (Ps. 22:15).
The Word spoke to Moses appearing before him “just as a man might speak with a friend” (Num. 12:8). But Moses wanted to see the One who was speaking with him openly and so it was said to him, “Stand in the cleft of the rock, and I will cover you with my hand. But when my splendor shall pass by, then you shall see my back parts but my face you shall not see; for man shall not see my face and live” (Ex. 32:20-22). This tells us two things: that it is impossible for man to see god; and that through the Wisdom of God man shall see Him, in the last times, in the cleft of a rock (that is in His coming as man). It was for this reason that He conferred with Moses face to face on the top of a mountain, when Elijah was also present as the Gospel tells us. And so in the end He made good the ancient promise.
*From Adversus Haereses
4.20.5-9