"Such illustrations and such images," says Athanasius, "has Scripture proposed, that, considering the inability of human nature to comprehend God, we might be able to form ideas even from these, however poorly and dimly, as far as is attainable." Orat. ii. 32, [amudros], vid. also [amudra]; ii. 17. (p. 92, under the heading "Economical Language")
If you go to the alphabetical listing of Newman's sermons, Anglican and Catholic, at the newmanreader.org website, you'll see several sermons with word "mystery" or "mysteries" in their title:
PPS2-18 Mysteries in Religion
DMC-13 Mysteries of Nature and of Grace
PPS4-19 The Mysteriousness of Our Present Being
DMC-14 The Mystery of Divine Condescension
PPS5-7 The Mystery of Godliness
PPS6-24 The Mystery of the Holy Trinity
There is nothing, according as we are given to see and judge of things, which will make a greater difference in the temper, character, and habits of an individual, than the circumstance of his holding or not holding the Gospel to be mysterious. (p. 292)
Till we {268} understand that the gifts of grace are unseen, supernatural, and mysterious, we have but a choice between explaining away the high and glowing expressions of Scripture, or giving them that rash, irreverent, and self-exalting interpretation, which is one of the chief errors of this time. [Newman then compares "men of awakened and sensitive minds" who are "led to place the life of a Christian, which "is hid with Christ in God," in a sort of religious ecstasy" with "sensible and sober-minded men, offended at such excesses" who think that the Gift of the Spirit "does nothing more than make us decent and orderly members of society"] . . .
For ourselves, in proportion as we realize that higher view of the subject, which we may humbly trust is the true one, let us be careful to act up to it. Let us adore the Sacred Presence within us with all fear, and "rejoice with trembling." Let us offer up our best gifts in sacrifice to Him who, instead of abhorring, has taken up his abode in these sinful hearts of ours. Prayer, praise, and thanksgiving, "good works and alms-deeds," a bold and true confession and a self-denying walk, are the ritual of worship by which we serve Him in these His Temples. How the distinct and particular words of faith avail to our final acceptance, we know not; neither do we know how they are efficacious in changing our wills and characters, which, through God's grace, they certainly do. All we know is, that as we persevere in them, the inward light grows brighter and brighter, and God manifests Himself in us in a way the world knows not of. In this, then, consists our whole duty, first in contemplating Almighty God, as in Heaven, so in our hearts and souls; and next, while we contemplate Him, in acting towards and for Him in the works of every day; in viewing by faith His glory without and within us, and in acknowledging it by our obedience. Thus we {270} shall unite conceptions the most lofty concerning His majesty and bounty towards us, with the most lowly, minute, and unostentatious service to Him.
If believers are really temples of the Holy Spirit, as St. Paul teaches, then it is practically sacrilegious for a baptized Christian not to be living the life of the Spirit. (p. 41)
People say that the doctrine of Transubstantiation is difficult to believe; I did not believe the doctrine till I was a Catholic. I had no difficulty in believing it, as soon as I believed that the Catholic Roman Church was the oracle of God, and that she had declared this doctrine to be part of the original revelation. It is difficult, impossible, to imagine, I grant;—but how is it difficult to believe? . . . What do I know of substance or matter? just as much as the greatest philosophers, and {240} that is nothing at all;"—so much is this the case, that there is a rising school of philosophy now, which considers phenomena to constitute the whole of our knowledge in physics. [Think of the "string theories" of recent years!] The Catholic doctrine leaves phenomena alone. It does not say that the phenomena go; on the contrary, it says that they remain; nor does it say that the same phenomena are in several places at once. It deals with what no one on earth knows any thing about, the material substances themselves. [Christ's Real Presence, Body, Blood, Soul, and Divinity in Holy Communion]
And, in like manner, of that majestic Article of the Anglican as well as of the Catholic Creed,—the doctrine of the Trinity in Unity. What do I know of the Essence of the Divine Being? I know that my abstract idea of three is simply incompatible with my idea of one; but when I come to the question of concrete fact, I have no means of proving that there is not a sense in which one and three can equally be predicated of the Incommunicable God.
Both are Mysteries we assent to without being able to prove them by human reason alone "through God's grace" and our cooperation with it. That of course, why Newman wrote--after years of wanting to write--An Essay in Aid of a Grammar of Assent as explained in this draft, unpublished preface, to demonstrate that it is not unreasonable to assent to mysteries like them.
Saint John Henry Newman, pray for us!