] IMHO JA does not often "slap you up side the head" with things. In this instance, I would assume that most of her contemporary readers were church goers and there were some things one just "knew". It was unnecessary to say it or make a "to do". They would automatically pick up on that fact.

Yes, you are right about her reluctance to hit readers over the head with things. However, to find out whether or not her readers would have picked up on things, we need to know what they believed at that time, and I'm not convinced that the readers when she wrote NA -- as opposed to when it was published -- would have picked up on it.

] That taboo was peculiar to that church and time (I don't know what they believe now a days).

I'm not at all sure that it was in 1798. A considerable change came over the Church of England, at least the more thoughtful members of it (in which I definitely include JA), as a result of the work of William Wilberforce and the Evangelical movement: it became a good deal more concerned with religious principles (such as keeping the sabbath) in that period.