Why did Russell see the necessity for dividing up Christians into "classes"? The distinction between the heavenly and earthly classes is essential to dispensationalism, but beyond that, Russell had three classes co-existing during the Gospel Age: justified believers, the Great Company, and the Little Flock. It must be remembered that Russell could not point to a previously existing church organization as possessing the "truth." Like William Miller before him, he had to appeal to Christians from various organizations. The "true church" had to be scattered invisibly among the denominations, waiting to be "harvested" at the end of the age. And, Christians had varying degrees of piety as well as belief. Russell had to find a way of saying that the 144,000, scattered among all churches and stretched out throughout the previous eighteen centuries, though being the cream of the crop, were not the only true Christians. Russell did this by stating that there was a class of Christians who had been justified because they believed in Jesus, but were never spirit-begotten. There was another class (the Great Company) who had been spirit-begotten, but had failed of their calling and yet had not been cast off completely.
But, by tampering with the doctrine of justification (see Part 2, my previous post), Russell created incalculable problems for his theology. When asked if his teachings had changed, he attempted to deny this. Nevertheless, justification had been the first rung in the ladder. If the Court of the Tabernacle pictured a condition that could not be scripturally proved to exist (mere tentative justification), how could Russell's other interpretations of the tabernacle (which, in turn, were re-worked into the Chart of the Ages) be relied upon? And a truth seeker would begin the quest by exposure to the "harmonious plan" of the first volume only to discover, in the sixth, that the harmony of the plan had been broken. This, after one had become throroughly enmeshed in the worldview, perhaps unable to exit.
A similar "Tabernacle problem" was Russell's inconsistency in identifying the antitypical Levites (as opposed to priests). Early on, Russell had stated that the justified believers (tentatively or otherwise) were the Levites because they never entered the Holy compartment but remained in the Court. But later, he claimed that the Great Company (who were spirit-begotten) were the Levites, as the Revelation (7:9-15) says that they serve God day and night "in his temple." Russell explained this by saying that the justified believers, as they lose their tentative justification, are expelled from the Court and go back to the Camp (the world). Thus, in the next age, they are no longer Levites. But the Great Company revert back to the Court. So while they (the Great Company) have been Priests during the Gospel Age, they return to being Levites in the future. The problem with this is that the Great Company is expected to enter upon a spiritual existence in the next age, not the human justified condition supposedly pictured by the Court. So Russell further confused his interpretation of the Tabernacle.
To conclude, there are difficulties in reading the SS. The original plan of the work was to conceal the realized eschatology from new readers, though this seems to have been didactic in nature rather a deliberate deception. But as inconsistencies in the theological system arose in succeeding years, the work was never thoroughly revised, leaving the unsuspecting reader to learn the early "errors" first and then having to re-learn the system. It is hoped this explanation helps any who may chance upon the volumes today and provide "a helping hand for Bible Students"! Finis.