1 Peter 1:12

There is one Son who accomplished the Father’s will and one human race in which the mysteries of God are accomplished, which angels long to behold.

Admirable sophists, and explorers of the sublimities of the unknown Father, and rehearsers of those super-celestial mysteries "which the angels desire to look into!".
For He has brought Himself, and has bestowed on men those good things which were announced beforehand, which things the angels desired to look into.
For there is the one Son, who accomplished His Father's will; and one human race also in which the mysteries of God are wrought, "which the angels desire to look into; "

But on the other side hear the Saviour: "I regenerated thee, who wert ill born by the world to death. I emancipated, healed, ransomed thee. I will show thee the face of the good Father God. Call no man thy father on earth. Let the dead bury the dead; but follow thou Me. For I will bring thee to a rest of ineffable and unutterable blessings, which eye hath not seen, nor ear heard, nor have entered into the heart of men; into which angels desire to look, and see what good things God hath prepared for the saints and the children who love Him."

"Which are now," he says, [1 Peter 1:12] "reported unto you by them that have preached the Gospel unto you." The old things which were done by the prophets and escape the observation of most, are now revealed to you by the evangelists. "For to you," he says, "they are manifested by the Holy Ghost, who was sent;" that is the Paraclete, of whom the Lord said, "If I go not away, He will not come." [John 16:7] "Unto whom," it is said, "the angels desire to look;" not the apostate angels, as most suspect, but, what is a divine truth, angels who desire to obtain the advantage of that perfection.

The mysteries of the more perfect Sacraments are of one kind; for the Scripture says, Eye hath not seen, nor ear heard, neither hath entered into the heart of man the things which God hath prepared for them that love Him. Of another kind are the things which the prophets have announced concerning future glory, unto whom it was revealed, and to whom the saints have preached the Gospel with the Holy Ghost sent down from heaven, which things the Angels desired to look into.

If the angels desire to look upon him, should not human beings be all the more afraid to despise him?

The angels long to look into these things because of the greatness of their love. They meditate on the Spirit and go on doing so forever, because love never comes to an end.

Some thought that the promise and the salvation of those who lived before the coming of Christ was inferior, on the grounds that those who saw him in the flesh and observed the miracles which he did had a greater reward. This is why Peter had to show that their impression was wrong. For how could Abraham be inferior to someone who lived after the coming of Christ, when he saw the day of the Lord and that everyone who would be made perfect by the Lord would depart into his bosom? How could Moses and Elijah be inferior, when they appeared with the Lord at his transfiguration, even though they did not see him in the flesh? Peter insists that it is not necessary to have seen Jesus in the flesh, for there were many unbelievers who did so, some of whom were bold enough to transgress the old covenant. And this has been said for the benefit of those who, even if they have not seen or heard what the Lord said in the flesh, nevertheless have a divinely inspired love for those things. If someone receives the salvation sought by the prophets, it is that which they all longed for at the end of time. For everything else was created by him, but this was not made by anyone. It was not possible for the holy angels or for any of the blessed rational creatures to partake of it beforehand, though they all longed to glimpse the things which would be revealed in the last days.

The angels both desire to see God and see him at the same time. For if they desired to see him but in such a way that their desire remained unsatisfied, they would be anxious and in suffering, but they are not. On the contrary, they are blessed and far removed from any kind of suffering or anxiety, since suffering and blessing do not go together.

Into whom angels long to look. It is indeed clear that so great the later glory of Jesus Christ, the man who suffered for us, followed, that even the angelic powers in heaven, though they are perfected in eternal happiness, not only rejoice to always look upon the eternal magnificence of the Deity, but also the glory of His assumed humanity. But it should be more diligently considered how it is said that angels desire to look into Him, since longing is not usually spoken of concerning that which we have, but that which we wish to have, for no one longs for what he has. How then do they desire to look into Christ, whose face they never cease to behold, unless the contemplation of the divine presence so beatifies the citizens of the heavenly homeland, that in an ineffable order for us, they are both always satisfied with the vision of His glory, and always hunger insatiably for His sweetness as if it were new? For, as blessed Pope Gregory, wonderfully distinguishing the delights of the heart and of the body, says, bodily delights, when not possessed, kindle a grave desire in themselves, but when possessed and consumed, immediately turn the eater into disgust through satiety. On the contrary, spiritual delights, when not possessed, are in disgust; when possessed, are in desire; and they are hungered for more by the one who eats them more, as they are consumed more by the one who is hungry. In those, appetite begets satiety, and satiety begets disgust; in these, appetite begets satiety, and satiety begets appetite. For spiritual delights increase desire in the mind while they satisfy it, because the more their flavor is perceived, the more it is known that they should be loved more. Moreover, what is said: "Into whom angels long to look," can also be rightly understood of the Holy Spirit, of whom it was promised: "Those who preached the Gospel to you in the Holy Spirit sent from heaven." For blessed Peter wished to refer this to the grace of divine pity, that He who is of such great majesty and glory, that His vision, as well as that of the Father and the Son (since indeed it is one and the same), is desired by angels in heaven, for the sake of human salvation sent the Spirit to earth, and infused it into the minds of the faithful to illuminate them.

To whom it was revealed that not to themselves, etc. Among the many secrets revealed to the prophets, when they diligently searched and inquired about the future salvation, it was also revealed to them that the same salvation would not be in their days, but rather in yours, who are born in the last days of the world. He says this to warn them to take care of the proffered salvation, which the prophets and earlier just ones loved so much, desiring to live in the world at the time when, immediately after departing from the world, it would be permitted to ascend to the heavenly kingdoms.

Through those who evangelized to you, etc. Previously he had said that the prophets foretold his sufferings and subsequent glories by the Spirit of Christ, and now he says the apostles declare the same to them by the Holy Spirit sent from heaven. Hence, it is clear that the same Spirit of Christ was in the prophets before, who later [was] in the apostles; therefore the same faith in the sufferings of Christ and the subsequent glories were preached by both to the people, [with] the former still expecting it to come, [and] the latter [declaring] it had already come; and by this, [it is evident] that there is one Church, of which one part preceded the Lord's advent in the flesh, [and] another part followed.

Concerning this salvation, the prophets who prophesied about the grace that was to come to you searched and investigated carefully, inquiring what person or time the Spirit of Christ within them was indicating when he testified in advance about the sufferings that would come upon Christ and the glories that would follow. It was revealed to them that they were not serving themselves but you, in the things that have now been announced to you through those who preached the Gospel to you by the Holy Spirit sent from heaven; into which angels long to see.
Since Peter had made mention of the salvation of the soul: however, this was not clear, but he brings forth the prophets who inquired and studied about it. For they inquired about future events: as Daniel did, who was called a man of desires by the angel, who gave him divine responses. (Dan. 9:23)
"inquiring what person or time the Spirit." To that, namely the consummation of the age: such as when the Jews, through various captivities, deviating from the perfect worship of God, were constituted towards the mystery of Christ. Note, however, that the spirit of Christ openly contemplates and discusses Christ. Moreover, this Spirit signified the afflictions of Christ, as Isaiah indeed says: "He was led like a sheep to the slaughter." (Isa. 53:7) And through Jeremiah: "Come, let us put wood in his bread." (Jer. 11:19) Moreover, the resurrection through Hosea: "On the third day we will be raised up before Him, and we will pursue Him to know the Lord; as the dawn is ready, we will find Him." (Hos. 6:2-3) However, it has been revealed not for Himself, but for you. Moreover, these two things he does with these words: Peter shows the providence of the prophets, and that those who are now called to the faith of Christ were foreknown by God before the foundation of the world. Indeed, through the foresight of the prophets, he leads them, so that they are not unbelieving regarding those whom the prophets greatly cared for, for their own benefit: for neither do wise sons despise their father's labors. Indeed, if they sought and found these things, when they would not benefit themselves, and committed the findings to books, as if transmitting an inheritance to us, we would act unjustly, he says, if we were negligent in these matters: therefore, do not show our gospel as empty, leading you who announce these things to you, as rejected and insignificant. Therefore, these things are from the providence of the prophets. Through this, however, that they were foreknown by God, they are terrified lest they present themselves as unworthy by divine foreknowledge or by His calling, but they urge and inspire each other, so that they may be deemed worthy of the gift of Christ.
"through those who preached the Gospel to you." If both the prophets and these have effectively worked through the Holy Spirit: indeed, those in prophecy, and these in the Gospel: nothing is different from the prophets for them: therefore, you must bring the same diligence that those who were obedient to the prophets, lest you be subjected to the same punishment as those who were unbelieving towards the prophets.
It must be noted, however, that through this mystery of the Trinity, Peter reveals: the Son and the Spirit when he says "the Spirit of Christ," but the Father in what follows, "from heaven." Moreover, when he says "from heaven," do not take it according to place, but understand it as excellence, because it especially applies to God.
"by the Holy Spirit sent from heaven." Peter speaks of what had happened on the day of Pentecost.
However, that "into which angels longed to see" means that which angels also desire; for this reason, even while such things were happening, they sang out of abundant joy: "Glory to God in the highest." (Luke 1:14)