I think that it makes more sense to see the phrase in I John as an introduction to the section that follows. In that case the criteria for making a distinction between truth and error is to test ideas in the community of love. The first interpretation of I John was the dominant method of discerning truth in the era of print dominance. The printing press technology encouraged an environment where disputation involves careful formulation of doctrinal statements. Every word can be carefully selected and then replicated on the printed page.
Uniformity becomes possible because everyone can have a printed copy of the same words.
Obviously, all the books in a theological library do not say the same thing. May of them were written specifically to show the errors in other books that are in the same library. Yet, taken as a whole, they represent the boundaries of theological discourse. However, compared to the variety and range of ideas that one finds on the Internet, a theological library is very limited. On the Internet we don't have publishers or theological librarians to determine the limits of theological discourse.