Fornication is sexual relations with another without them being married.
Since there is no formal tie, they could separate and go on to sex with someone else, ad-nauseum and very many people do that.
I tend to think fornicators are those who have multiple sexual partners when unmarried and commit adultery when married when they engage in sex outside of the marriage.
But if they are committed to each other and stay together, then they are married, and they also need to formally sanctify the relationship to their church as a witness to all the congregation as godly examples and get married officially. They must also in the eyes of God, since He sees them as a couple, remain together, and follow all the marriage obligations in the NC. Otherwise, reject them, remove them from the church, if they will not repent, Gods says they are evil persons.
1 Corinthians 7
36 But if any man thinks he is behaving improperly toward his [
d]virgin, if she is past the flower of youth, and thus it must be, let him do what he wishes. He does not sin; let them marry.
Just because someone says they believe in Christ, does not mean they are born again as His people.
People have to bear good fruit to God (demonstrate fruits of the Spirit in their life) or they are not saved, their relationship is just fake news.
Which Christ are they believing and following?
Maybe a false Christ?
Matthew 24
4 And Jesus answered and said to them: “Take heed that no one deceives you.
5 For many will come in My name, saying, ‘I am the Christ,’ and will deceive many.
Matthew 3
9 and do not think to say to yourselves, ‘We have Abraham as
our father.’ For I say to you that God is able to raise up children to Abraham from these stones. 10
And even now the ax is laid to the root of the trees. Therefore every tree which does not bear good fruit is cut down and thrown into the fire. 11 I indeed baptize you with water unto repentance, but He who is coming after me is mightier than I, whose sandals I am not worthy to carry. He will baptize you with the Holy Spirit [
a]and fire.
God knows those who are His,
2 Timothy 2:19, ESV: But God’s firm foundation stands, bearing this seal: “The Lord knows those who are his,” and, “Let everyone who names the name of the Lord depart from iniquity.”
When there is no repentance from doing evil, there is no godly induced sorrow and I question someone's claim of salvation.
We are NOT to have anything to do with those kinds of people, who scripture calls evil
2 Corinthians 7:10
For
godly sorrow produces repentance
leading to salvation, not to be regretted; but the
sorrow of the world produces death.
1 Corinthians 5
Immorality Must Be Judged
9 I wrote to you in my epistle not to [
f]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.
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.”