The "Eye that sees us all" Xavier sees at the end is God.
The preacher tells him it's Satan, but it would make more sense for it to be God. God is thought to be The Omniscient (he sees us all), and He rules the Universe, so Him being at the center of the Universe makes sense.
In fact, Xavier's description of his final vision — "A light that glows, changes... and in the center of the universe... the eye that sees us all" — is highly reminiscent of Dante's final vision of God◊ at the end of The Divine Comedy.
The "Eye that sees us all" Xavier sees at the end is some other poor bastard who invented the same thing.
Only that one didn't have the guts to go through with self-blinding, and wound up seeing even more than Xavier. Including Xavier himself.
◊ at the end of The Divine Comedy.
