What makes you think that the man in Corinthians was actually a Christian?
Nothing that I can see in the passage indicates he was a genuine beliver that I can see.
We know that this sin was abhorrent to the Pagans and even the culture and the Jewish community.
1 Corinthians 5:1
Sin in Relationships (5:1–6:20)
Paul Confronts Sexual Immorality and Spiritual Pride
1 I can hardly believe the report about the sexual immorality going on among you—something that even pagans don’t do. I am told that a man in your church is living in sin with his stepmother.
Now it seems that the church accepted this. So now they have allowed a little leaven into the church.
We find that Paul is not actually addressing the sinful nature of people outside of the church. He says
1 Corinthians 5:9-10
Immorality Must Be Judged
9 I wrote to you in my epistle not to keep company with sexually immoral people. 10 Yet I certainly did not mean with the sexually immoral people of this world, or with the covetous, or extortioners, or idolaters, since then you would need to go out of the world.
So it has nothing to do with sexually immoral people outside of the church because if it was then they would not be in the world and preach the gospel.
The later verses in chapter 5 is Paul clarifying what he meant.
1 Corinthians 5:9
Immorality Must Be Judged
9 I wrote to you in my epistle not to keep company with sexually immoral people.
Then he clarifies
1 Corinthians 5:11-13
11 But now I have written to you not to keep company with anyone named a brother, who is sexually immoral, or covetous, or an idolater, or a reviler, or a drunkard, or an extortioner—not even to eat with such a person.
12 For what have I to do with judging those also who are outside? Do you not judge those who are inside? 13 But those who are outside God judges. Therefore “put away from yourselves the evil person.”
Verse 11 does not say a brother, it's a named brother.
Basically someone claims to be a brother (genuine beliver) yet is living in unrepentant sin. Therefore discipline him, excommunicate him from you in the hope that he will repent of this sin.
The premise I'm trying to make here is that we cannot assume this man was saved to begin with.
The purpose of this whole chapter is
To convict people in the church not to be fooled by their spiritual state.
1 Corinthians 10:12
12 Therefore let him who thinks he stands take heed lest he fall.
To protect the spiritual health of believers
Galatians 2:4
4 And this occurred because of false brethren secretly brought in (who came in by stealth to spy out our liberty which we have in Christ Jesus, that they might bring us into bondage),
To not give unberlievers an excuse to slam the church
1 Peter 2:12
12 having your conduct honorable among the Gentiles, that when they speak against you as evildoers, they may, by your good works which they observe, glorify God in the day of visitation.
As I say nothing in this chapter suggests the man was saved to begin with.
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