Calvin on the Need for the Spirit to Accompany the Word

A simple external manifestation of the Word ought to be amply sufficient to produce faith, did not our own blindness and perverseness prevent it. But such is the proneness of our mind to vanity, that it can never adhere to the truth of God, and such its dullness, that it is always blind even in his light. Hence without the illumination of the Spirit the Word has no effect; and hence also it is obvious that faith is something higher than human understanding.

The Word is, in regard to those to whom it is preached, like the sun which shines upon all, but is of no use to the blind. In this matter we are all naturally blind; and hence the Word cannot penetrate our mind unless the Spirit, that internal teacher, by his enlightening power make an entrance for it.

--John Calvin, Institutes (3.2.33-34)