One wonders how the Apostles would have dealt with a Church-member who advocated for laws encouraging adultery, infanticide, or homosexuality.
According to scripture, you’d meet privately to allow the person to “save face”.
If that didn’t work, then you would not allow the person to affiliate with your church-community. Hence the justification for excommunication. The notion that people “excommunicate” themselves is a lame attempt to pass a leadership responsibility to a the individual...it’s cowardice.
Acts 5:1-11 But a man named Ananias, with his wife Sapphira, sold a piece of property, and with his wife's knowledge he kept back for himself some of the proceeds and brought only a part of it and laid it at the apostles' feet. But Peter said, “Ananias, why has Satan filled your heart to lie to the Holy Spirit and to keep back for yourself part of the proceeds of the land? While it remained unsold, did it not remain your own? And after it was sold, was it not at your disposal? Why is it that you have contrived this deed in your heart? You have not lied to man but to God.” When Ananias heard these words, he fell down and breathed his last. And great fear came upon all who heard of it. The young men rose and wrapped him up and carried him out and buried him.
After an interval of about three hours his wife came in, not knowing what had happened. And Peter said to her, “Tell me whether you sold the land for so much.” And she said, “Yes, for so much.” But Peter said to her, “How is it that you have agreed together to test the Spirit of the Lord? Behold, the feet of those who have buried your husband are at the door, and they will carry you out.” Immediately she fell down at his feet and breathed her last. When the young men came in they found her dead, and they carried her out and buried her beside her husband. And great fear came upon the whole church and upon all who heard of these things.
And then there are these instructions from Paul about immorality in the church.
1 Corinthians 5:1-13 It is actually reported that there is sexual immorality among you, and of a kind that is not tolerated even among pagans, for a man has his father's wife. And you are arrogant! Ought you not rather to mourn? Let him who has done this be removed from among you.
For though absent in body, I am present in spirit; and as if present, I have already pronounced judgment on the one who did such a thing. When you are assembled in the name of the Lord Jesus and my spirit is present, with the power of our Lord Jesus, you are to deliver this man to Satan for the destruction of the flesh, so that his spirit may be saved in the day of the Lord.
Your boasting is not good. Do you not know that a little leaven leavens the whole lump? Cleanse out the old leaven that you may be a new lump, as you really are unleavened. For Christ, our Passover lamb, has been sacrificed. Let us therefore celebrate the festival, not with the old leaven, the leaven of malice and evil, but with the unleavened bread of sincerity and truth.
I wrote to you in my letter not to associate with sexually immoral people—not at all meaning the sexually immoral of this world, or the greedy and swindlers, or idolaters, since then you would need to go out of the world. But now I am writing to you not to associate with anyone who bears the name of brother if he is guilty of sexual immorality or greed, or is an idolater, reviler, drunkard, or swindler—not even to eat with such a one. For what have I to do with judging outsiders? Is it not those inside the church whom you are to judge? God judges those outside. “Purge the evil person from among you.”