“We are taught in this passage that the knowledge of Christ must be sought from the Scriptures. Those who imagine what they like about Christ will ultimately have nothing but a shadowy ghost in His place.
First then, we must hold that Christ cannot be properly known from anywhere but the Scriptures. And if that is so, it follows that the Scriptures should be read with the aim of finding Christ in them.
Whoever turns aside from this object, even though he wears himself out all his life in learning, will never reach the knowledge of the truth. For how can we be wise apart from the Wisdom of God?
Moreover, as we are commanded to seek Christ in the Scriptures, so He declares in this passage that our work will not be fruitless, for there the Father bears witness to His Son in such a way that He will manifest Him to us beyond all fought.
But what hinders most men is that they look at them only carelessly and as it were in passing. But it needed the utmost application, and so Christ commanded them to search diligently for this hidden treasure.
Accordingly, the abhorrence for Christ which the Jews feel, who have the Law constantly in their hands, must be imputed to their laziness. For the brightness of God’s glory shines clearly in Moses, but they want to have a veil to obscure that brightness.
‘By the Scriptures,’ of course, is here meant the Old Testament. For Christ did not first begin to be manifested in the Gospel; but the One to whom the Law and the Prophets bore witness was openly revealed in the Gospel.”
–John Calvin, The Gospel According to St John: 1-10, Calvin’s New Testament Commentaries, Vol. 4; trans. T.H.L. Parker, Ed. David Torrance and Thomas Torrance (Grand Rapids, MI: Eerdmans, 1961), 139. Calvin is commenting on John 5:39.