This means He is not known through human reason.
It also means He is not known through human religion.
The Bible often engages with other religions. Never does it assume that such religions have any revelatory insights to offer.
Numbers 33:50-53; Deut 7:1-6; 12:1-3; 29:16-18; 32:15-21; Psalm 96:4-5; 106:35-40; Isaiah 41:21-24; 44:6-26; Jeremiah 16:19-21; Romans 1:23-25; 1 Corinthians 8:4-6; 10:20.
We know that an idol is nothing at all in the world and that there is no God but one. For even if there are so-called gods, whether in heaven or on earth (as indeed there are many "gods" and many "lords"), yet for us there is but one God, the Father, from whom all things came and for whom we live; and there is but one Lord, Jesus Christ, through whom all things came and through whom we live. (1 Corinthians 8:4-6)
- A religious person may speak eloquently about their "lords" and their "gods" - aspects may seem similar to the Living God, yet they are not speaking of the God who has made Jesus Christ the point of contact. They are speaking about something else - not the Father, Son and Holy Spirit. For more on this see my posts on Allah.
It is instructive that none of the prophets ever speak with Assyrians, Moabites, Baal worshippers etc and say "Yeah, Yahweh - He's like a cross between Asherah and Baal - but, like, bigger." Or "Of course you know El Elyon through the primitive religious spark latent within you. But let me now introduce to you the Son of the Most High, His name is Yahweh. If you like Elyon, you'll love the LORD!" Yet how many Christian apologists function with just this kind of methodology. Anyway, mustn't get side tracked...
Big point: human religion is not a stepping stone to the living God. It does not yield partial knowledge that can then be built upon towards a knowledge of Jesus.
Ok, maybe world religions don't have an angle on God. But surely there is one religion in the world that does. Don't the Jews have revelation of the Living God? They share our Scriptures (three quarters of them!!). Don't they know at least some truth about the One True God?
Well, what did Jesus think about that? Let's look at John 5:37-46:
The Father who sent me has Himself testified concerning Me. You have never heard His voice nor seen His form, nor does His word dwell in you, for you do not believe the One He sent. You diligently study the Scriptures because you think that by them you possess eternal life. These are the Scriptures that testify about Me, yet you refuse to come to Me to have life...
..."But do not think I will accuse you before the Father. Your accuser is Moses, on whom your hopes are set. If you believed Moses, you would believe Me, for he wrote about Me. But since you do not believe what he wrote, how are you going to believe what I say?"
Jesus acknowledges these men's diligent study of the Scriptures. Yet He denies outright that they have any knowledge of the Father or the Son. They have sought to by-pass revelation in Christ and their very means of attempting this will be their damnation.
To the person who does not come to Christ, the Scriptures leave them utterly ignorant of the Living God. Even 'diligent' study of the Bible leaves a person utterly lost unless they are drawn to the central character of the Bible - Jesus. No part of the Bible - not Moses, not the Prophets, nothing - reveals God outside of Jesus.
This of course has many implications for how we read the Old Testament. If you've read my blog for any length of time you'll know my position on Christ in the Old Testament. To put it in John 5 terms, Moses believed in and proclaimed Christ. Whatever you believe about such things, it is enough presently to note that Jesus does not consider the Bible, in any part, to be a Christ-less revelation of God.
Now none of this is to be construed as an anti-Jewish sentiment. Nor is it some chronological snobbery against the ancient world. It's not something that Christians can laugh off as an error belonging to another people or another time. If any human action towards God could lay hold of true salvation or knowledge - then surely it would be Jewish religion and Jewish Scripture. Their privileges are "much in every way." (Rom 3:2) Yet the failure of Jewish religion should serve as a stark warning for those who would seek to grasp the things of God through some supposed Christian religion. Doesn't the experience of the Pharisees in John 5 warn us too? Shouldn't we too be wary of our diligent study? Their error is repeated in Christians time and again. We have these witnesses to Christ (Scripture and also sacraments etc). Yet rather than receive them with empty hands and be led away from ourselves and to the One in Whom all salvation and all knowledge of God is complete, we attempt to use them as building blocks towards our own salvation and our own synthesized theology.
To know God we must abandon the attempt to build, to strive, to ascend. It is all a given in Christ. Human religion, even biblical religion - when used rather than received - will produce only ignorance. We must begin again with Jesus at the foundation.
So we've rejected reason and religion. Next time we'll see how creation is not the way to a true knowledge of God either.