For the hidden things of the Son — the title/inscription of the ninth psalm, cited by Jerome as referring to the hidden wisdom of Christ
Pope Clement VIII, Jerome's Prefaces, On Worship
II. JEROME TO PAULINUS.— Jerome cites the psalm's title as referring to Christ, in whom all treasures of wisdom and knowledge are hidden
"the title of the ninth psalm is inscribed, 'For the hidden things of the Son,' in which all the treasures of the wisdom and knowledge of God are hidden."