Just as God will judge every believer by what they built on that foundation ( salvation ) laid by Jesus Christ 1 Corinthians 3:10 as any that sows to the flesh ( and heresy is one of them ) that defiles the temple of God, and remains defiled by the time the Bridegroom comes, that believer will be left behind to die per 1 Corinthians 3:16-17 although still saved 1 Corinthians 3:15.
Revelation 2:18-25 is a warning to that church to repent or else be cast into the bed of the coming great tribulation and be judged by their works with death as those that did not resurrect at the great tribulation rapture event, will have to wait Revelation 6:9-11 along with believers left behind to be resurrected after the great tribulation Revelation 7:9-17 & Revelation 20:1-6
Since God commanded the church through Paul to excommunicate any unrepentant brother, for this purpose, in 1 Corinthians 5:4-5, you may see why those unrepentant saints & former believers are left behind. Indeed, 1 Corinthians 5:8-11 shows how the Lord will hold the Marriage Supper in heaven in the same way church fellowship & communion should be held down here.
So that is why we are not to judge anyone as not saved or not a true Christian, but as not abiding in Him & His words in correcting them to lead them to repentance so as to live that reconciled relationship with God the Father through Jesus Christ.
