The Mystical Doctor on revelation

The principal reason why in the law of Scripture the enquiries that were made of God were lawful, and why it was fitting that prophets and priests should seek visions and revelations of God, was because at that time faith had no firm foundation, neither was the evangelical law established; and thus it was needful that they should enquire of God and that He should speak, whether by words or by visions and revelations, or whether by figures and similitudes or by many other ways of impressing His meaning.

But now that faith is founded in Christ, and, in this era of grace, the evangelical law has been made manifest, there is no reason to enquire of Him in this manner, nor for Him to speak or to answer as He did then. For, in giving us, as He did, His Son, which is His Word -- and He has no other -- He spoke to us all together, once and for all, in this single Word, and He has no occasion to speak further. And this is the sense of that passage which St. Paul begins, when he tries to persuade the Hebrews that they should abandon those first manners and ways of converse with God which are in the law of Moses, and should set their eyes on Christ alone: In the old days God spoke to out fathers in many ways and by many means, through the prophets; now at last in these times He has spoken to us, with a Son to speak for Him (Heb. 1:1). And this is as though he had said: That which God spake of old in the prophets to our fathers in, in sundry ways and diverse manners, He has now, at least, in these days, spoken to us once and for all in the Son. Herein the Apostle declares that God has been, as it were, dumb, and has no more to say since that which He spake aforetime, in part, to the prophets, He has now spoken altogether in Him, giving us the All which is His Son. Wherefore he that would now enquire of God or seek any vision or revelation, would not only be acting foolishly, but would be committing an offence against God, by not setting his eyes altogether on Christ, and seeking no new thing or aught beside. And God might answer him after this manner, saying: If I have spoeken all things to thee in My Word, which is My Son, and I have no other word, what answer can I now make to thee, or what can I reveal to thee which is greater than this? Set thine eyes on Him alone, for in Him I have spoken and revealed to thee all things, and in Him thou shalt find yet more than that which thou askest and desirest. For thou askest locutions and revelations, which are the part; but if thou set thine eyes upon Him thou shalt find the whole; for He is My complete locution and answer, and He is all My vision and all My revelation; so that I have spoken to thee, answered thee, declared to thee and revealed to thee, in giving Him to thee as thy Brother, Companion and Master, as ransom and as reward. For since that say when I descended upon Him on Mount Tabor, saying "This is my beloved Son, in whom I am well pleased; hear ye Him" (Matt. 17:5), I have left off all these manners of teaching and answering and I have entrusted this to Him. Hear Him; for I have no more faith to reveal, neither have I any more things to declare. For if I spake aforetime, it was to promise Christ; and if they enquired of Me, their enquiries were directed to petitions for Christ and expectancy concerning Him, in Whom they should find every good thing (as is now set forth in the teaching of the Evangelists and the Apostles), but now, any who would enquire of Me after that manner, and desire me to speak to him or reveal aught to him, would in a sense be asking Me for Christ again, and asking me for Me for more faith, and be lacking in faith which has already been given in Christ; and therefore would be committing a great offence against my beloved Son; for not only would he be lacking in faith, but he would be obliging Him again first of all to become incarnate and pass through life and death. Thou shalt find naught to ask of Me or to desire of Me, whether revelations or visions; consider this well, for thou shalt find that all has been done for thee and all has been given to thee, -- yea, and much more also -- in Him.

St. John of the Cross, Ascent of Mount Carmel, Book II, Ch. 22, sect. 3 (Peers translation).

For a similar idea expressed by St. Thomas, see Sum. Theol. II-II, 1, 7.