John Chrysostom — as excerpted in the Catena Aurea on John 1:18

“(Hom. xv. [xiv.] 3) But what hath He declared? That God is one. But this the rest of the Prophets and Moses proclaim: what else have we learnt from the Son Who was in the bosom of the Father? In the first place, that those very truths, which the others declared, were declared through the operation of the Only Begotten: in the next place, we have received a far greater doctrine from the Only Begotten; viz. that God is a Spirit, and those who worship Him must worship Him in spirit and in truth; and that God is the Father of the Only Begotten.”