In a doctoral dissertation De recta ratiocinatione (1686), Gisbert Wessel Duker claimed that "the divinity of Scripture cannot be demonstrated except by reason." During the promotion session at the University of Franeker, the legal scholar Ulrik Huber (1636-1694) objected to this statement by reading from a copy of the Institutes he had in hand what John Calvin had written about the necessity of the testimony of the Holy Spirit. This article traces Huber's use of Calvin in various writings published during the ensuing controversy, most notably the De concursu rationis et Sacrae Scripturae (1687). That Huber used him as an authority is significant because he was a legal scholar, and not a theologian, who appealed to Calvin in support of the need for the Holy Spirit and for humble piety in order to counter tendencies in the (legal) philosophy and theology of his day to grant natural human reason a normative status in religion. © 2011 Koninklijke Brill NV, Leiden.