And there's no record they weren't believers.
That's just rationalization to make the event fit your theology.
Paul also gives instructions in Corinthians for the man who was in immorality with his mother-in-law.
God dealt with sin at the cross but that does not mean we can sin without consequence since then. That is demonic thinking. God will NOT bless a person, a people, or a nation that turns their back on Him and chooses to live in unrepentant sin.
1 Peter 4:17 For it is time for judgment to begin at the household of God; and if it begins with us, what will be the outcome for those who do not obey the gospel of God?
For I will be merciful to their unrighteousness, and their sins and their lawless deeds I will remember no more.” 13 In that He says, “A new covenant,” He has made the first obsolete. Now what is becoming obsolete and growing old is ready to vanish away.Hebrews 8:12-13.
“This is the covenant that I will make with them after those days, says the Lord: I will put My laws into their hearts, and in their minds I will write them,” 17 then He adds, “Their sins and their lawless deeds I will remember no more.” 18 Now where there is [e]remission of these, there is no longer an offering for sin.Hebrews 10:16-18.
When Scripture repeats itself, it usually means it is important to pay attention. THRICE scripture specifically repeats that the New Covenant means that sin is no longer the issue with God anymore. We are JUSTIFIED by grace - JUSTIFICATION means "JUST AS IF I never did it". That is EXACTLY what the New Covenant is saying and EXACTLY how God treats those who have received Christ and his perfect and eternal sacrifice.
You want to bring back the condemnation of the law but your doctrine is totally unscriptural under the Gospel of the Grace of Christ and carries with it a double curse (Gal. 1:8-9).
Scripture calls the ministry of the condemnation of the law, the ministry of death (2 Cor 3:7). We are not to be ministers of the condemnation and death of the law but ministers of reconciliation.
And all things are of God, who has reconciled us to himself by Jesus Christ, and has given to us the ministry of reconciliation; 19 that is, that God was in Christ, reconciling the world unto himself, not imputing their trespasses unto them; and has committed unto us the word of reconciliation. 20 Now then we are ambassadors for Christ, as though God did beseech you by us: we pray you in Christ's stead, be ye reconciled to God. 21 For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him.2 Corinthians 5:18-21.