In The Stones of Venice Ruskin celebrates the apparent roughness of the Venitian Gothic and its unfinished look over the perfection and harmony of Renaissance architecture. Ruskin claims that Gothic buildings look that way because, in their construction, workers and architects were granted a larger freedom from past models and struggled to make them unique and personal creations. The project of Ruskin's book as a whole is therefore to rescue the reputation of Gothic architecture from the disrepute that Renaissance canon had thrown it into:
the word Gothic became a term of unmitigated contempt, not unmixed with aversion. From that contempt, by the exertion of the antiquaries and architects of this century, Gothic architecture has been sufficiently vindicated ; and perhaps some among us, in our admiration of the magnificent science of its structure, and sacredness of its expression, might desire that the term of ancient reproach should be withdrawn, and some other, of more apparent honourableness, adopted in its place.
In the chapter quoted in your question, Ruskin directly links the Gothic style to morality and God. The author urges us to consider the savageness of Gothic architecture, not merely as a result of the Northern-European environmental conditions in which it was born, but as "an index of religious principle". Ruskin links this idea with his previous argument on the apparent imperfection of the Gothic stating that Christianity recognises the value of personal contributions and prizes the imperfection of individual expression over the perfect models of the Renaissance. This acknowledgment of loss of power of human minds tends in the end to affirm God's greater powers. So Gothic architecture not only recognises the value of individual expression,
it confesses its imperfection, in only bestowing dignity upon the acknowledgment of unworthiness. That admission of lost power and fallen nature, which the Greek or Ninevite felt to be intensely painful, and, as far as might be, altogether refused, the Christian makes daily and hourly, contemplating the fact of it without fear, as tending, in the end, to God’s greater glory. Therefore, to every spirit which Christianity summons to her service, her exhortation is : Do what you can, and confess frankly what you are unable to do ; neither let your effort be shortened for fear of failure, nor your confession silenced for fear of shame. And it is, perhaps, the principal admirableness of the Gothic schools of architecture, that they thus receive the results of the labour of inferior minds ; and out of fragments full of imperfection full of imperfection, and betraying that imperfection in every touch, indulgently raise up a stately and unaccusable whole.