Sentence:  He was thinking; but the glory of the song, the swell from the great organ, the clustered lights, […], the height and vastness of this noble fane, its antiquity and its <WSD>strength</WSD>—all these things seemed to have their part as causes of the thrilling emotion that accompanied his thoughts. 
Meaning of the word marked with <WSD> tags:  The quality or degree of being strong.