Is A Tale of Two Cities primarily a social critique or a theological vision (or a ghostly mixture of both)?

I would argue that it's primarily a social critique. In A Tale of Two Cities, Dickens compares British and French society and finds the former, for all its many faults, superior. In the sedition trial of Charles Darnay, for example, Dickens highlights the toxic atmosphere of paranoia that existed at that time in Britain while revolution raged on the other side of the English Channel. Yet despite that, Darnay is acquitted of the trumped-up charges laid against him; the rule of law has prevailed (with a little help from Sydney Carton).
Compare this with the system of revolutionary tribunals that exists in France and which almost sends Charles to his death. Once again, Sydney saves Charles's neck, but only by putting his own on the block, both literally and figuratively. Whatever faults British society may have, Dickens appears to be saying, at least it has some measure of order and stability. This is more than can be said for Revolutionary France, with all its chaos, turmoil, and bloodshed.