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.
There are more cities in Asia than these, but he is commanded to write to those taught by him and who had received the faith of Christ. For those who did not believe and turned away from the saving word, how could anybody even advise them?
I was in the spirit: These words mean that that he saw a vision that was not understandable by the senses nor seen with physical ears or eyes, but with a prophetic sense. Isaiah spoke concerning this spiritual hearing when he told “me a learned tongue, that I should know how to uphold by word him that is weary: he wakened in the morning, in the morning he wakened my ear, that I may hear him as a master. The Lord God has opened my ear. (Is. 50:4-5)”
[AD 220] Tertullian on Revelation 1:10