Isaiah did not see the face of God. It was a vision. Nobody saw God’s face until they beheld Jesus, the Second Person of the Trinity incarnate.
And the Word became flesh and dwelt among us, full of grace and truth; we have beheld his glory, glory as of the only Son from the Father. (John 1:14)
The longing to see God was a constant theme through the Old Testament, and no one was permitted to see God’s face and live until the advent of Christ.
That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked upon and touched with our hands, concerning the word of life (1 John 1:1)
We see the face of God every day in the Mass if we want. when the priest raises the Eucharist and says, “Behold!” We see the face of God “in the flesh” every day when we look at the Eucharist. That is something Moses and the Old Testament patriarchs longed to do.
At the coming of Christ, man was finally able to look upon God with his eyes - not in a vision or in a dream or the back of God’s head, but the real, living God. We get to do it every day.
-Tim-