“This passage teaches us that if we would know Christ, we must seek Him in the Scriptures. Anyone who imagines Christ as he will, gets nothing but a mere blur.
So, we must first hold that Christ is known rightly nowhere but in Scripture. If this be so, our chief purpose in reading the Scriptures must be to arrive at a right knowledge of Christ.
Whoever turns aside from this aim, even though he wear himself out with learning all his life, will never arrive at truth; for what wisdom can we attain apart from the wisdom of God?
Moreover, since we are commanded to seek Christ in the Scriptures, He declares that our zeal in this matter shall not be in vain; for the Father Himself testifies that in them He shall certainly reveal His Son to us.
Many are deprived of this blessing, because they neglect reading the Scriptures, or do it cursorily and superficially. But it deserves utmost attention that Christ Himself commands us to probe deeply into this hidden treasure.
It was sheer apathy that led the Jews, who had the law in their very hands, to abhor Christ. The glory of God shone brightly in Moses, but they put up a veil and darkened it.
In this place, Scripture means obviously the Old Testament. It is not true that Christ appears first in the gospel.
It is rather that after the witness of the Law and the Prophets, He appeared in the gospel for everyone to see.”
–John Calvin, Calvin: Commentaries, Ed. Joseph Haroutunian (Philadelphia: Westminster Press, 1958), 105. Commenting on John 5:39.