They did not know that he spoke to them about the Father. Hence we see how stupid those men are whose understandings are possessed by Satan. Nothing could be more plain than that they were summoned to the judgment-seat of God. But what then? They are altogether blind. This happens daily to other enemies of the Gospel; and such blindness ought to instruct us to walk with fear. [1]

Footnotes

1 - "A cheminer en crainte."

They understood not - They knew not, or they were unwilling to receive him as a messenger from God. They doubtless understood that he meant to speak of God, but they were unwilling to acknowledge that he really came from God.

(11) They understood not that he spake to them of the Father.
(11) Even the contempt of Christ results in his glory: and this thing his enemies will eventually feel to their great pain.

They understood not that he spake to them of the Father. That sent him, and who was true and faithful to all he had said, whether in a way of promise, or threatening; such was their stupidity, that they did not know that he meant God the Father by him that sent him, so deriving his mission and doctrine from him; their hearts were made fat, and hardened, and their eyes were blinded. The Vulgate Latin version reads, "they did not know that he said, God was his Father"; and so Beza's most ancient copy, and another exemplar of his.

They understood not - That by him that sent him he meant God the Father. Therefore in John 8:28-29 he speaks plainly of the Father, and again claims the Divine name, I AM.