Paul also dealt with unrepentant brothers for why churches are to excommunicate them to lead them to repentance per 1 Corinthians 5:1-13.Paul dealt with all kinds of problems but still fellowshipped with immature churches.
If you can't find literally anyone to fellowship and obey hebrews 10:25 with, and you see apostates behind every bush, then the problem might be in the mirror.
Paul also addressed those who fell away from the faith as this iniquity was happening back in his days in 2 Thessalonians 2:1-15 and He continued to address the disorderly that will be damned by issuing the commandment to withdraw per 2 Thessalonians 3:1-7 but he pointed out that in withdrawing from them, it is not to treat them as an enemy, but to admonish them as brothers still 2 Thessalonians 3:14-15 because they are His.
Believers that think they can change the minds of an erring & unrepentant church should consider Paul's calling them out here.
2 Corinthians 6:14 Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness? 15 And what concord hath Christ with Belial? or what part hath he that believeth with an infidel? 16 And what agreement hath the temple of God with idols? for ye are the temple of the living God; as God hath said, I will dwell in them, and walk in them; and I will be their God, and they shall be my people. 17 Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing; and I will receive you. 18 And will be a Father unto you, and ye shall be my sons and daughters, saith the Lord Almighty.
It is like a Catholic who believes they are saved but they are staying in the church to show their faith in Him so others may believe, but in the eyes of others, they cannot see their faith in Him that they are saved when to those that believe by staying within the church & doing the works within is how they are saved per their Catholic Catechism of #846. Also #846 with #847 is why today, the Pope tells Catholics not to convert unbelievers but non-Catholics Christians because the CC believes unbelievers are saved regardless. Such thinking makes one wonder why Jesus came at all. Would you agree that would be a time to walk away? Believe it or not, that is how bad the other churches are today when you consider the distant Presbytery ruling over churches that the pastor would be risking losing his job if they went against an erring Presbytery.