Paul had heard certain things regarding the church in Corinth and he knew that they were really happening. Here he writes by revelation (in accordance with the information being revealed to him via the holy spirit within him) regarding one of these subjects.
I Corinthians 5:1 and 2:
5:1Fornication among you is wholly heard-of, even fornication of-this-kind which (is) not-even among the Gentiles – so-as-for someone to have (his) father's wife! - .2and you are having been puffed-up2 and have not rather mourned in order that the (person) having done this work may be lifted out-from your midst!
Paul comes straight to the point: a great amount is presently being heard of how there is fornication among you holy-people (sanctified-people, Christians, saints, children of the only true God), and such fornication which is not even done among the Gentiles (people of nations other-than Israel/Judah who have not believed what God has said regarding Himself and His son, the Lord Jesus Christ, and therefore are not holy-people, they do not have holy spirit-life and therefore are not children of God, they are not Christians).
The basic meaning of "fornication" is "wrongful sexual activity/relations." The Gentile fornication that Paul is referring to is that-which was generally practiced by some Gentiles during their gatherings as a form of worship and service to their idols or devil-gods, right there in the middle of it all, even on the altar! This, of course, was indeed contrary to what Paul had been teaching the Corinthian holy-people now that they had received the gift of holy spirit from the only true God and were able to worship Him in spirit and in truth.
But what kind of fornication could be so contrary that not even the Gentiles would do it during their worshipping and serving their "gods"? This was specifically in such a manner as for someone to have the wife/woman of his father. The son had sexual relations with his father's wife/woman - who was either the son's natural mother or his step-mother! One of the young men in the assembly, the church, in Corinth did this as part of his "worship" and "service" supposedly to the only true God! He brought over some of his old idolatrous practices which he used-to perform towards idols and devil-spirits (demons), and not only their practices but he heightened that form of idol worship and service to a more devilish degree by having sexual relations with his father's woman which was expressly forbidden by God. He did this idolatrous act knowingly and willingly within the fellowship of the holy-people who were to worship and render-service towards God as God instructs. This was like slapping the face of God bringing idolatry and devil-spirit worship into His church, among His children in the disguise of doing it all for God. No wonder this sort of fornication was not even among those Gentile people who had refused what the true God had said and instead chose to believe the lie of the devil.
However, the other holy-people in this fellowship of God's children were puffed-up when this sort of thing happened and were remaining puffed-up (literally: they were physically or naturally breathing and swelling up; figuratively: they were inflated in their fleshly thinking regarding the situation). And, they had not more-so mourned (lamented) for the purpose and result that the person having done this work (deed, action) would be taken and carried-out from the middle of them, from the gathering of holy-people who ought to worship and serve God as God asks them to worship and serve Him.
Also, did you notice that Paul called this act of fornication against God "work"? It certainly would have taken a lot of energy, action, work, for this act of "worship" and "service" to be done. Even though this person put effort, energy, work into his action, it was very much contrary to God and all that God asks His children to do as worship and service to and for Him.
[Reference: Leviticus 18:8; Deuteronomy 22:30 and 27:20; Ezekiel 16:15-34; Matthew 5:32, and 15:16-20; John 4:24; Acts 15:20 and 29; Romans 12:1 and 2; I Corinthians 6:18; II Corinthians 7:8-12; Revelation 17:1-19:2. Note: verbs with a superscript 2 (2) immediately following them indicate the "perfect" tense - details are provided in the "Relevant Notes" link of this study.]
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