Irenaeus was the earliest Christian theologian, born about 130 AD and bishop of Lyons. From his Against Heresies:
Just as those who see the light are in the light and share in its brightness, so those who see God are in God and receive His splendour. Now the splendour of God gives life; those who see God, therefore, receive life. That is why He who is uncontainable and incomprehensible and invisible presented Himself to be seen and comprehended and contained by men. He wanted to give life to those who receive and see Him. For just as His greatness is beyond measure, so His goodness is beyond expression. By that goodness, having been seen, He bestows life on those who see Him. It is impossible to live without life, and there is only life by participation in God. But to participate in God is to know God and to enjoy His goodness. Men will, therefore, see God that they may live, and by the vision they will be made immortal and attain God.