Holiness is, AFAICS, the inconceivable "Unlikeness", or Incomparability of God, as compared with any or all created beings. Because nothing but God is perfectly like God. So nothing but God can perfectly and fully receive & reciprocate the Love of God - a Love that is Itself God.
God is not simply Good or Goodness - God is intolerably, awfully, terribly Good. God is therefore ferociously possessive: He cannot be satisfied with anything short of total, unreserved, unlimited love that comes from the whole heart, the whole mind, the whole soul and the whole strength. Because that is how God Himself loves - God "puts all that He is" into the love He gives.
St John of the Cross says it well:
“As eyes weakened and clouded by humours suffer pain when the clear light beats upon them, so the soul by reason of its impurity suffers exceedingly when the Divine Light really shines upon it. And when the rays of this pure light strike upon the soul to expel its impurities, the soul perceives itself to be so unclean and miserable that it seems as if God had set Himself against it, and it were set against God.
The soul seeing distinctly in this bright and pure light, though dimly, its own impurity, acknowledges its own unworthiness before God and all creatures”.
- St John of the Cross, Dark Night of the Soul, Book 2.5.6
“All the goodness of the world together, in comparison with the infinite goodness of God, is wickedness rather than goodness, for “none is good but only God’....
All the wisdom of the world, all human cunning, compared with the infinite wisdom of God, is simple and supreme ignorance, “for the wisdom of this world is foolishness with God.”....
“All the liberty and power of the world, compared with the liberty and power of the spirit of God, is but supreme slavery, wretchedness and captivity....””
- St John of the Cross, Ascent of Mount Carmel, Book 1.4.5,6,7.
Or there is this:
“Miserable man that I am, what fellowship hath my perverseness with my uprightness ?....Thou art truly good, I wicked; Thou art full of compassion, I am hard of heart; Thou art holy, I am miserable; Thou art just, I am unjust; Thou art light, I am darkness; Thou art life, I am dead; Thou art medicine, I am sick; Thou art sovereign truth, Ι [am] utter vanity”.
- St Augustine of Hippo (354-430), Soliloquy, chapter 2.
Or this:
“It is the sight of God revealed to the eye of Faith that makes us hideous to ourselves from the contrast which we find ourselves to present to that great God at whom we look. It is the vision of Him in His infinite gloriousness, the All-Holy, the All-beautiful, the All-perfect, which makes us sink into the earth with self-contempt and self-abhorrence. We are contented with ourselves till we contemplate Him.”
- St John Henry Newman (1801-90), Sermons preached on Various Occasions.
The book from which these quotations are taken is “The Good Pagan’s Failure”, by Rosalind Murray, published by The Catholic Book Club in 1939. These quotations are given on pages 31-32.
Immediately after giving these quotations, she writes:
“These are not artificial sentiments nor pious efforts to edify. They are attempts on the part of these great Christians to express in words what perhaps cannot be expressed, an apprehension of the fact of God.”
“The Good Pagan and the Christian may often act in the same way, make the same moral judgements; they may both be virtuous, honourable and just, they may both be charitable and unselfish, they may be equally so, but the Pagan is satisfied, the Christian not at all. The Pagan has attained his aim of the “good life”, he is a man of honour, a good citizen, a good human being, he knows it and is content.
The Christian, on the other hand, measures himself by quite another standard, not in human excellence, but in relation to God. He is a citizen of another city. The idea of God becomes, to him, more real, more absorbing, in proportion as he himself draws nearer to him. He is living his life as it were in a new dimension, in which the goodness he has attained seems negligible, non-existent, in comparison to the goodness he apprehends.
It is indeed notable that the greater the Christian, the more nearly totalitarian in nature, the more does he suffer from a sense of sin, of imperfection, of inadequacy. It is easy to give examples of this truth:
“He who loves any other thing than God, makes light of Him, because he puts into the balance with Him that which is infinitely beneath Him...There is nothing in the whole world to be compared with God, and therefore he who loves any other thing together with Him, wrongs Him. And if this be true, what does he do, who loves anything more than God ?”
- St John of the Cross (1542-91), Ascent of Mount Carmel, Book 1.5.4
In the OT, the ferocity of God's Holiness is dangerous to human beings - even lethal.
For the Sumerians, holiness seems to have been an awe-inspiring or terrifying "radiation", "emitted" by holy things like deities, temples, and kings. There was therefore a clear distinction between the "Sacred" & the "Profane". This outlook was shared by later cultures, such as that of Israel.
When Moses wears a veil over his face after seeing God, it is because he has been "infected" with God's Holiness as a result of being with Him. The veil functions as a Hazmat suit, to protect the Israelites who have not seen God.
The notion of holiness as necessarily having an ethical aspect comes much later. Holiness, unlike moral goodness (however excellent) is a numinous quality - it comes to man from the Sacred "beyond".
And some forms of holiness are anything but humane and reasonable: human sacrifice is an encounter with the Holy, but is not humane.
This book is a classic on the subject of the Holy:
The Idea Of The Holy : Otto, Rudolf : Free Download, Borrow, and Streaming : Internet Archive