In the sixth chapter of the first book of the Institutes, Calvin discusses the “The need of Scripture, as a guide and teacher, in coming to God as a Creator”. Here, he reflects on why God preserved his Word in the written word:
“For if we reflect how prone the human mind is to lapse into forgetfulness of God, how readily inclined to every kind of error, how bent every now and then on devising new and fictitious religions, it will be easy to understand how necessary it was to make such a depository of doctrine as would secure it from either perishing by the neglect, vanishing away amid the errors, or being corrupted by the presumptuous audacity of men. It being thus manifest that God, foreseeing the inefficiency of his image imprinted on the fair form of the universe, has given the assistance of his Word to all whom he has ever been pleased to instruct effectually, we, too, must pursue this straight path, if we aspire in earnest to a genuine contemplation of God; — we must go, I say, to the Word, where the character of God, drawn from his works is described accurately and to the life; these works being estimated, not by our depraved judgement, but by the standard of eternal truth. If, as I lately said, we turn aside from it, how great soever the speed with which we move, we shall never reach the goal, because we are off the course. We should consider that the brightness of the Divine countenance, which even an apostle declares to be inaccessible, (1 Tim. 6:16), is a kind of labyrinth, — a labyrinth to us inextricable, if the Word do not serve us as a thread to guide our path; and that it is better to limp in the way, than run with the greatest swiftness out of it.” (Calvin, Institutes, 1.6.3.)
Wander away from “the Word” and off the course of sound doctrine and one is bound to devise “new and fictitious religions”… religions that may even mention Jesus and use Christian terms.