So something independent does need to exist in order to be our shared reference point but that does not mean that this independent thing is non-mental.
Human beings genetically have similar filters. We are programmed in a sense with the same sensory input.
The different senses are very foreign to eachother. Sight and hearing draw from the same source, but the information is filtered in a radically different way. Thus, the experience from sight and hearing are alien to eachother.
Besides information being filtered through our senses, it is further filtered through our cognition.
The shared reference point is almost universally mental, or a conception.
In this sense, The Ultimate Reality as a concept to be communicated is noumenon, but what it actually is referring to cannot be noumenon. A conception or mental construct is not God, otherwise the essence of this conception is not fulfilled. The essence of this conception is that it is not a conception, because the.conception itself is an arrow pointing to something else.
If The Ultimate Reality is noumenon, it does not fulfil the basic requirement of what The Ultimate Reality is in essence. Mind is a greater reality than noumenon, and mind itself is contingent on The Ultimate Reality.
Our ancient theologians knwing this problem said "In the beginning was the Word, and the Word was with God, and the Word was God.", not to say that God was a word, or noumenon, but to make clear that this Word is an image of what it represents, and it must be respected that when we use this image or icon, we are refering to the prototype of the icon.
Believe He is who He says He is. Get it?
What God is in essence. What The Ultimate Reality is in essence.
It is The Holy Spirit, or Spirit of Truth which comes from The Father, through The Son, and allows us to speak of The Father in The Son.
This I am speaking of is a great part of the Mystagogy of the faith. We acknowledge thst the only way to The Father is through The Son with The Holy Spirit as helper.
And this is something that distinguishes Orthodox Christianity from other monotheist faiths. By acknowledging the icon, we aren't mistaking a conception of God as God!