grammar - Are "as of yet", "as yet" and "yet" all synonymous?

This comes from Paradise Now from April D. De Conick:

But if there be any as yet unfit to be called a son of God, let him press to take his place under God's Firstborn, the Word, who holds eldership among the angels, an archangel as it were.

I stumbled upon "as yet" and found it to be a bit perplexing and it seems to mean the same thing as "yet", which is a bit confusing. Are all three phrases synonymous 100% of the time?

The above seems to mean "But if there was anyone yet unfit to be called a son of God, let him be called God's Firstborn's son, the Word, who holds eldership among the angels, an archangel so to speak."

Answer

Depending on context, yet and as yet can mean different things.

Merriam-Webster has a specific sense of as yet that it defines:

3 : NEVERTHELESS, HOWEVER
as yet : up to the present or a specified time
// there are as yet few clues
— Sharon Kingman

This is in contrast to the primary sense of yet:

1 a : in addition : BESIDES
// gives yet another reason

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In your specific example, both as yet and yet are very likely to mean the same thing, which is:

But if there be any still unfit to be called a son of God . . .

However, as yet could have a subtly different implication of something that is more likely to change in the short-term, or something that is, surprisingly, still as it is.

There is no cure for cancer yet.
There is as yet no cure for cancer.

There is a difference in terms of the specific grammar used in this particular case, but there is also a very subtly different sense of of expectation in the second version, and a very subtle implication that a cure is less far off.

Having said that, for all intents and purposes they mean the same thing.