The short answer is: it means "because they were hard devout Christians".
This kind of appositive construction gives an explanation for the actions or behaviour of the phrase or noun in apposition, for example:
Jonathan, the hard worker he is, kicked it up a notch and offered to fix the truck for a much cheaper rate than a mechanic
Which could be rewritten as:
Jonathan, being the hard worker he is, kicked it up a notch and offered to fix the truck for a much cheaper rate than a mechanic
or even:
Because Jonathan is such a hard worker, he kicked it up a notch and offered to fix the truck for a much cheaper rate than a mechanic
It would seem that your example is missing a comma. Consider rewriting it as:
Her very father and mother, hard devout Christians that they were, who had never spared the rod in the upbringing of the children, deferred to him then, and heard his words with respect.