Yes, I am. It wasn't simply a dropped-s typo because you began your sentence with was. The crusades, as a movement, are never referred to in the singular in any of the literature. So I find it hard to believe anyone with even minimal immersion in that literature could begin a sentencing with "was the crusade". My point? You invoke the crusades without knowing a damn thing about them.
Was the Crusades linked to a popular interpretation of a major religion? Are we in a grammar school here? Geez. Technically, although "was the motive for the Crusades linked to a popular interpretation of a major religion" is grammatically correct, you can still counter by saying that there was no single, unified "motive." But I think the meaning of my sentence is clear. So instead of nitpicking, why don't you just answer the question and tell us whether you think the "Crusades" was linked to a popular interpretation of a major religion.
No, it had nothing to do with an 'interpretation' of Christianity, in the sense that some people interpreted Christianity as requiring the Crusades, while others interpreted as prohibiting such a movement. The Crusades were political. They were also certainly justifiable - by both secular and and Christian morality.