Sentence:  The fact <WSD>is</WSD> incontestable : that liberal world of Unitarian Boston was narrow-minded , intellectually sterile , smug , afraid of the logical consequences of its own mild ventures into iconoclasm , and quite prepared to resort to hysterical repressions when its brittle foundations were threatened . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)