See subjects. And there are two of them goddmamnit, even though the second one is interesting relative to the first. So no getting on my case for bad OP. [You know who you are.]
Basically, born of a Jewish mother. The “who’s a Jew” problem could still apply, but nothing like today, which leads to the second subject in this OP:
“Observance” could be, I don’t know, making sure to show up in Jerusalem and do your thing at the Temple, to Essene and other cults, to the way-Hellenized (Romanized?–I really don’t know) who doesn’t give a damn except for what may be necessary to negotiate with the theocratic nomenklatura.
A contemporary sort-of parallel, I think, would be the population of Saudi Arabia, from the rural peasant to the governing class. The split between “religion”/culture Judaism and “nation”/government today obviously is different than the many Islamic cultures still operant today–and I wonder how similar even that is to Judea at that time.