The meaning of 'relative to equally virtual members'
At first I thought it might be a typo for "equally virtuous, but after consideration I now think they mean...
almost a particular thing or quality
Cambridge
"relative to equally virtual members "
The discussion seems to be about an entity which existed only in the mind(s) of certain philosophers, and was expected to be established at some time in the future, just a distant future. Hence the almost.
However, the 2 ideas seem to be dichotomous to the writer, and possibly contradictory.
The quoted text is a little unclear in its intent. Perhaps a careful re-reading of the surrounding context would shed more light, but I don't see myself going there for something which might be better explained at another SE site.