Wrong, The Northerners worship mostly the Old Gods, they are First Men so they don't worship the Andals's Seven.The Stark children worship both because Ned allowed their mother to give them also that type of religious education, but even Sansa (probably the more distant to Stark values) prefers to worship the Old Gods.In Westeros there is not an official state religion like in medieval Europe, the various people living there are free to worship whatever religion they wish, indeed the Northern wedding done by the Old Gods in ADWD has the same value as a wedding done in the sight of the Seven.
Robb is King In The North, he has only one vassal family (House Manderly) that follows the Faith of the Seven, while the rest of his vassals and people worship the Old Gods.Indeed, that scene was a disgrace.