9 Hear me, therefore, my fellow-servant, my friend, my brother; give ear for a moment that I may tell you how you are to walk in the holy scriptures. All that we read in the divine books, while glistening and shining without, is yet far sweeter within. “He who desires to eat the kernel must first break the nut.” “Open thou my eyes,” says David, “that I may behold wondrous things out of your law.” Now, if so great a prophet confesses that he is in the darkness of ignorance; how deep, think you, must be the night of misapprehension with which we, mere babes and unweaned infants, are enveloped!
Now this veil rests not only on the face of Moses, but on the evangelists and the apostles as well. To the multitudes the Saviour spoke only in parables and, to make it clear that His words had a mystical meaning, said:— “he that has ears to hear, let him hear.” Unless all things that are written are opened by Him “who has the key of David, who opens and no man shuts, and shuts and no man opens,” no one can undo the lock or set them before you. If only you had the foundation which He alone can give; nay, if even His fingers were but passed over your work; there would be nothing finer than your volumes, nothing more learned, nothing more attractive, nothing more Latin.
Source: Letters (New Advent)