There is the selfsame difference in respect of the soul's corporeality, which is (perhaps) invisible to the flesh, but perfectly visible to the spirit. Thus John, being "in the Spirit" of God, beheld plainly the souls of the martyrs.
He says that he was taken up in the spirit, that is, that he was raised up to the secret things of God, in order that he might see those things which he was to speak. Moreover, he says that he did not enter into the heights of heaven in a bodily manner, but that he entered in the spirit, recalling this word, “No one has ascended into heaven except he who has descended, namely the Son of man, who is in heaven.” The holy apostle Paul also says that he was taken up, but in what way? He says, “Whether in the body or out of the body, I do not know, God knows.” He writes that he had been taken up into ecstasy, in the spirit. But since the day of the Lord is mentioned in this passage, when he says that he had been taken up in the spirit, he is indicating that he had been cleansed of any work of a profane kind. For, on the Lord’s day the apostle could only devote himself to divine things and holy duties.… Concerning the preachers of the gospel, it is written, “Cry aloud, do not cease, lift up your voice as a trumpet.” And concerning the words “behind me,” the prophet said, “And they shall hear a voice from behind of one teaching.” Let all humanity be exalted to whatever degree of sanctity, in comparison to the holy acts of God and to the divine words, it can by no means ever stand as an equal before his presence and face. But our flesh, weighed down by a certain weakness, is instructed, as it were, from behind by the words of God. Therefore, in saying “behind me” he indicates the weakness of his human nature.
I was in the Spirit on the Lord’s day. He also indicates a suitable time for the spiritual vision. For Scripture often expresses the fixed limits of causes, as often of place, body, or air, so also of time. For the angels visit Abraham at noon, Sodom in the evening. Adam feared at the voice of the Lord walking in the afternoon. And Solomon received wisdom at night, which he would not keep.
I was in the spirit on the Lord's day. Here we need to start speaking about species and genus individually. John's spirit did not leave his flesh altogether when he saw this, because his body could not have survived without spirit; but, being in ecstasy, he could not feel or understand anything through his body. Note also that he says he was in the spirit not on the Sabbath, but on the Lord's day, because the old law, which produced death, had already passed, and the new one, which gives life, had become well-known with Christ's Resurrection. Allegorically, the Church is also shown to be the Lord's day, because, in the elect, it mortifies the deeds of the flesh by the spirit [Rom. 8:13] so that it may rise in the spirit in the conduct of a new life. Whence this saying of the apostle: You are not in the flesh, but in the spirit. [Ibid. 9] And heard behind me a great voice, as of a trumpet, saying: What thou seest, write. What one should know is that just as John heard it in the spirit, so was the voice spoken to him spiritual. We should however ask the question of what back a spirit could have to be able to hear a voice behind him. It seems then that the face of that spirit was the very force of divine contemplation, and its back, obliviousness of present matters. Therefore he heard the voice in the things he had forgotten, the purpose of this being to make him turn his mind's face towards them and write in a book everything he saw; as if the voice were saying to him, “Those things which you are beginning to see in the spirit of God, either have been done or must be done here, where you hear me sounding without sound, that is on earth.” With the words What thou seest, write, it is as if he were saying “what thou art about to see,” for he had not seen anything yet. Allegorically, the Church hears a voice concerning its union with Christ, behind it, from the law and the prophets, like this: They shall be two in one flesh. [Gen. 2:24] This voice, whether with regards to the Church or with regards to John, is rightly called great, because it speaks of the highest mysteries, and because of that it is compared to a trumpet and spread by the mouths of the preachers; whence it is said to one of them, Lift up thy voice like a trumpet. [Is. 58:1] This voice, through John, tells the Church to write what it sees, that is, to keep it in mind. And send to the seven churches, to Ephesus, to Smyrna, to Pergamus, to Thyatira, to Sardis, to Philadelphia, and to Laodicea. We need to say how the interpretations of these names tally with the universal Church. Ephesus translates to “my will” or “my counsel”: and whose counsel, but that of the one who casteth away the counsels of princes, but his counsel standeth forever? [Ps. 32:10-11] Moreover, the Church is the will of God, to which it is said through the prophet, Thou shalt be called My will. [Is. 62:4] Note also that will pertains to love, while counsel pertains to correction. Indeed, someone is the will of God when they submit to the Creator not out of fear of punishment, but willingly; whereas if someone wants to join neither for fear nor for love, they are given a speech of correction through the action of divine mercy so that they may abandon their error and receive the counsel of salvation. Smyrna translates to “their canticle,” that is the elect's, and that canticle is the new commandment. Therefore the Church sings this canticle every day when it fulfills the new commandment by loving God and neighbor. Pergamus translates to “separating their horns”: and whose horns but Christ's and the Devil's? So by Christ's horns is represented his kingdom, that is the Church, and by the Devil's horns, it is also his kingdom that is represented, that is the wicked. Hear therefore the separation of the horns: I will break all the horns of sinners, and the horns of the just shall be exalted. [Ps. 74:11] Thyatira means “enlightened”: and what other enlightened one are we to understand, but the one to whom it is said by Isaiah, Arise, be enlightened, O Jerusalem? [Is. 60:1] It is appropriate for the Church to be called enlightened, so that it should remember that it used to be darkness; whence the apostle to its members: You were heretofore darkness, but now light in the Lord. [Eph. 5:8] Sardis means “for the prince of beauty,” and “fitted” or “prepared” is implied. The prince of beauty is the one about whom the Psalmist says, beautiful above the sons of men. [Ps. 44:3] And who is prepared for him, if not the one about whom the prophet says to the same prince, The queen stood on thy right hand, in gilded clothing; surrounded with variety? [Ibid. 10] Philadelphia translates to “saving the inheritance for the Lord.” From this it is right to understand the Church of the elect, which strives to save itself with divine help. Indeed it is about this inheritance that it is said, I will give thee the Gentiles for thy inheritance. [Ps. 2:8] Laodicea translates to “tribe lovely to the Lord” or “they were in vomit”: this interpretation shows the good mingled with the wicked in the Church.
There are many cities in Asia, but he himself commands to write to those who have been taught by him and have already accepted the faith of Christ. For what could one advise to those who are unbelieving and have turned away from the saving word?
When John said, "I was in the Spirit" indicates a vision not perceptible to the senses, nor seen by the flesh or eyes, but rather by prophetic insight; concerning these spiritual perceptions Isaiah said: "He gave me understanding early on, He gave me an ear to hear, and the instruction of the Lord opened my ears." (Isa. 54:4)
[AD 220] Tertullian on Revelation 1:10