18 I am he that liveth, and was dead; and, behold, I am alive for evermore, Amen; and have the keys of hell and of death.
[AD 735] Bede on Revelation 1:18
And I have the keys of death and Hades. He says, not only have I conquered death by resurrection; but I also have dominion over death itself. He also granted this to the Church by breathing the Holy Spirit upon it, saying, "Whose sins you forgive, they are forgiven," and so forth.

[AD 804] Alcuin of York on Revelation 1:17-18
And when I had seen him, I fell at his feet as dead. In this passage John signifies Christ's limbs, and the angel signifies Christ specifically. Therefore John falls as dead at the angel's feet because the Church follows Christ's footprints to die to the world. It is appropriate to say that he is as dead, because even if the Church dies to the world and to vices, it still lives to God. And he laid his right hand upon me, saying: Fear not. The right hand of the angel in this place represents the preachers. Hence it is said by Moses, In his right hand a fiery law, [Deut. 33:2] that is, in the holy preachers the teaching of the Holy Spirit. It is not mentioned at all what the angel did with his hand laid upon him, but we deduce with a certain reasoning that he lifted up from the ground either John or the Church in him. Now, can it be that the Church was put away from the imitation of the sufferings of Christ? No. Therefore the angel laying his hand upon John who lay at his feet as dead and raising him from the ground, is the Redeemer Lord lifting up the Church which imitates his sufferings from sorrow of the heart to the hope of future glory by encouraging it through the holy preachers, as in The sufferings of this time are not worthy to be compared with the glory to come, that shall be revealed in us. [Rom. 8:18] He fittingly says Fear not, as if the Lord were saying to the Church, “Receive confidence from above, and drive human fear away.” I am the First and the Last, and I was alive, and dead, and behold I am living for ever and ever. Here it is made clear that this angel specifically bore the person of the Lord Jesus Christ, whom all this corresponds to. And have the keys of death and of hell. By the keys is represented the divine judgment’s power, which no one can escape, but it shuts everyone in like a key, according to this: He shutteth, and no man openeth; [Rev. 3:7] by death is meant the one about whom it is written, By the envy of the devil, death came into the world; [Wis. 2:24] and by hell are represented those who belong to him. So with one set of keys the Devil is released to tempt the Church, and with the other he is bound; whence it is written, God is faithful, who will not suffer you to be tempted above that which you are able. [1 Cor. 10:13]

[AD 990] Oecumenius on Revelation 1:17-19
It is the habit of the holy prophets when seeing a vision to be struck with amazement and to exhibit human weakness. Divine matters far exceed human affairs, and surpass them in ways past all comparison. We know that this is what Joshua the son of Nun felt when he saw the commander-in-chief of the battle-line of the Lord, as did Daniel, “the man of desires,” in the visions seen by him.

Therefore I fell at his feet as though dead, says the evangelist, struck with amazement by the sight. But he laid his right hand upon me, saying, Do not fear. Saint John would not have been able to stay alive as a result of being struck by the vision, if the saving right hand of the Son of God had not touched him, which could effect so many marvels with a single touch.

And he says to me, I am the first and the last, as if he said, “I am the one who came to be with you in the flesh for the salvation of all of you at the end of the age, though I am the first and ‘the first-born of all creation.’ So how can you suffer any evil from my appearance? For if while living and being a fountain of life for you I died and again came to life after trampling on death, how can you who have life because of me and my vision, ever become dead?

And if I also have the keys of Death and of Hades, so that those whom I wish I may put to death or preserve alive, and if I could send them down to Hades and bring them up in accordance with what has been written about me, and mine are, as the prophet says, “the ways of escape from death,” I would not dispatch my worshipers and disciples to an untimely death.”

Since, therefore, he will not die, he says, write what you have seen, both what is and what is going to take place. In saying what is, he means both past and present events, and in saying, what is going to take place, he means future events. Of the things seen in the vision by the holy one, some had already taken place. Even if he had gone back as far as he could, these events would not have preceded the beginning. So he mentioned what is; but the argument will go on to show some events that were past and some which were still to take place.

[AD 2020] Douglas Wilson on Revelation 1:18
In the time before the Messiah came, the expectation of the godly was to die and go to Sheol. Jonah (most likely) actually died and cried out to God from the depths of Sheol (Jon. 2:1). The psalmist expected that Sheol would swallow him up (Ps. 18:5; 86:13; 116:3).

In the parable of Lazarus and the rich man, they both died and went down to Hades. In that parable, Hades was divided in two by a vast chasm. The side where Lazarus was had the name of Abraham’s bosom (Luke 16:23), while the rich man was in torment in Hades. Nevertheless, it was possible for communication to occur across the chasm.

In our text [Matt. 12:40], Jesus said that He was going to be three days and nights in the heart of the earth. But He also told the thief on the cross that He would be with him in Paradise that same day (Luke 23:43). So then, Abraham’s bosom was also known as Paradise. To the Greeks, this went by the name of Elysium. This is where Jesus went, and preached across the chasm.

The Greek word for the lowest pit of Hades, the worst part, was Tartarus. This word is used once in the New Testament (without any redefinition, mind). Peter tells us this: “For if God spared not the angels that sinned, but cast them down to hell [Tartarus], and delivered them into chains of darkness, to be reserved unto judgment” (2 Peter 2:4).

While in Hades, the Lord preached. But the preaching was not “second chance” preaching. Rather the word used is one used for heralding or announcing, not the word for preaching the gospel. “By which also he went and preached unto the spirits in prison; Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water” (1 Pet. 3:19–20). The Lord was announcing their final defeat to the “sons of God” and Nephilim both. And this, incidentally, tells us how momentous the rebellion at the time of the Flood actually was. Thousands of years after their definitive defeat, Jesus went to them to announce their final defeat.

The Bible teaches us that Jesus is the king of all things. The devil is not the ruler of Gehenna—Jesus is. The lake of fire was prepared for the devil and his angels (Matt. 25:41). It is a place of torment for the devil. Furthermore, Jesus holds the keys to Hades as well. “I am he that liveth, and was dead; and, behold, I am alive for evermore, Amen; and have the keys of hell [Hades] and of death.” (Rev. 1:18). Jesus, not the devil, is the King of Hell. Jesus, not the devil, is the Lord of Hades.

When the Lord rose from the dead, He led captivity captive (Eph. 4:8)—all the saints in the Old Testament who had died and gone to Abraham’s bosom were transferred when Paradise was moved (Matt. 27:52). And by the time of Paul, Paradise was up (2 Cor. 12:4). So if you had lived in the Old Testament, you would have died and gone down to Sheol/Hades. But the part of Hades that contained the saints of God has been emptied out, and now when God’s people die, what happens? To be absent from the body is to be present with the Lord (2 Cor. 5:6, 8). We still go to Paradise, but Paradise itself has been moved into the heavens.