The opening and final paragraphs of the above referenced article (which do not appear above) define such ones as those that have known the truth and been blessed by the truth but have turned away from it -- like the dog returning to its vomit. In this context 'the truth' is not the organization as is now taught -- but Biblical truth, and about the ransom in particular.
The language is very similar to what is used by the Society today. But today the language is all loaded. Russell was referring to Scriptural Apostasy rather than any sort of Organizational Apostasy.