5 These things did the prophets set forth in a prophetical manner; but they did not, as some allege, [proclaim] that He who was seen by the prophets was a different [God], the Father of all being invisible. Yet this is what those [heretics] declare, who are altogether ignorant of the nature of prophecy. For prophecy is a prediction of things future, that is, a setting forth beforehand of those things which shall be afterwards. The prophets, then, indicated beforehand that God should be seen by men; as the Lord also says, “Blessed are the pure in heart, for they shall see God.” But in respect to His greatness, and His wonderful glory, “no man shall see God and live,” for the Father is incomprehensible; but in regard to His love, and kindness, and as to His infinite power, even this He grants to those who love Him, that is, to see God, which thing the prophets did also predict.
“For those things that are impossible with men, are possible with God.” For man does not see God by his own powers; but when He pleases He is seen by men, by whom He wills, and when He wills, and as He wills. For God is powerful in all things, having been seen at that time indeed, prophetically through the Spirit, and seen, too, adoptively through the Son; and He shall also be seen paternally in the kingdom of heaven, the Spirit truly preparing man in the Son of God, and the Son leading him to the Father, while the Father, too, confers [upon him] incorruption for eternal life, which comes to every one from the fact of his seeing God.
For as those who see the light are within the light, and partake of its brilliancy; even so, those who see God are in God, and receive of His splendour. But [His] splendour vivifies them; those, therefore, who see God, do receive life. And for this reason, He, [although] beyond comprehension, and boundless and invisible, rendered Himself visible, and comprehensible, and within the capacity of those who believe, that He might vivify those who receive and behold Him through faith. For as His greatness is past finding out, so also His goodness is beyond expression; by which having been seen, He bestows life upon those who see Him.
It is not possible to live apart from life, and the means of life is found in fellowship with God; but fellowship with God is to know God, and to enjoy His goodness.
Source: Adversus haereses (New Advent)