Although the expression The most high
is quite archaic, it is still used, specially in religious literature to make a reference to God or something related to heaven. It is a resource used in comunication in order to enphasize how much God is important compared to any other gods. However, according to the modern English, the current and correct spelling is the highest
You should read: https://www.englishexperts.com.br/forum/comparative-x-superlative-explicacao-detalhada-t210.html
So we have:
igh [with capital letters] or The H
ighest.The Highest should be enough in this case, but it is not an isolated case
- O Altíssimo.
The highest [god; mountain;]
- O deus superior; a montanha mais alta.
Glory / hosanna in the h
- Glória / hosana nas alturas.
So, my conclusion is the highest can be a noun and also an adjective.
(1) Trask, R.L. Dictionary of Grammatical Terms in Linguistics. Taylor & Francis. 2013.
(2) McMenomy, Bruce A. Syntactical Mechanics: A New Approach to English, Latin, and Greek. University of Oklahoma Press, 2014.