not-so-definite question

I'd put it this way:

1. «A righteousness from God»
It equals to «A righteousness that comes from God»

Now, it's in direct analogy to, for example, «A book that is interesting», which in turn semantically equals «An interesting book». Of course, indefinite article here, because we want to refer to an object (righteousness, book) from a multitude (all righteousnesses, all books) with certain properies (thigns coming from God, interesting things).

2. «A righteousness that...»
See above.

Hope this helps.

guideline in regard to the use of an indefinite article before an countable noun

righteousness is not countable!

a here is not any different from a with other uncountable nouns.  In other words, the same guidelines apply.
Whether the noun is concrete or abstract does not necessarily make any difference.

a wine  = a certain kind of wine
a sugar = a certain kind of sugar
a righteousness = a certain kind of righteousness
an intelligence = a certain kind of intelligence

CJ

Thank you.

CalifJim's emphasis acknowledged. I was writing it without due attention and spelled it wrong.

It should be an uncountable.