Have you looked into what the outer darkness is? It never says it is cast into the lake of fire in the end. I believe it’s where the vessels of dishonor go. The place is mentioned three times in Mathew. It seems to me that it is believers who go there, not unbelievers.
Considering the difference between the kingdom of light and the world, those cast out would be into outer darkness.
I’m hoping it’s a purgatorial type place, as in the parable of the man who had his debt forgiven but then refused to forgive the debt of another, so he was thrown into a prison from which he was told he would not come out until he paid the last farthing.
In a way, when a believer refuses to forgive another believer, he is not welcomed to the Marriage Supper because there will be no grudges there when He brought us all there by His righteousness & His faithfulness. So to hold a grudge at the Supper Table with someone there is not happening. That is why that person will not be there but be outside that reception and thus paying for not being ready to go there.
So I can see why Jesus warned believers after teaching His disciples how to pray.
Matthew 6:9 After this manner therefore pray ye: Our Father which art in heaven, Hallowed be thy name. 10 Thy kingdom come, Thy will be done in earth, as it is in heaven. 11 Give us this day our daily bread. 12 And forgive us our debts, as we forgive our debtors. 13 And lead us not into temptation, but deliver us from evil: For thine is the kingdom, and the power, and the glory, for ever. Amen. 14 For if ye forgive men their trespasses, your heavenly Father will also forgive you: 15 But if ye forgive not men their trespasses, neither will your Father forgive your trespasses.
That is why we are to forgive others as Christ has forgiven us. And we need His help to forgive them anyway when they are not asking for forgiveness because Jesus had set the example for us on the cross in suffering grief wrongfully in following Him in forgiving others for they know not what they do.
Luke 23:33 And when they were come to the place, which is called Calvary, there they crucified him, and the malefactors, one on the right hand, and the other on the left. 34 Then said Jesus, Father, forgive them; for they know not what they do. And they parted his raiment, and cast lots. 35 And the people stood beholding. And the rulers also with them derided him, saying, He saved others; let him save himself, if he be Christ, the chosen of God.
1 Peter 2:19 For this is thankworthy, if a man for conscience toward God endure grief, suffering wrongfully. 20 For what glory is it, if, when ye be buffeted for your faults, ye shall take it patiently? but if, when ye do well, and suffer for it, ye take it patiently, this is acceptable with God. 21 For even hereunto were ye called: because Christ also suffered for us, leaving us an example, that ye should follow his steps: 22 Who did no sin, neither was guile found in his mouth: 23 Who, when he was reviled, reviled not again; when he suffered, he threatened not; but committed himself to him that judgeth righteously: 24 Who his own self bare our sins in his own body on the tree, that we, being dead to sins, should live unto righteousness: by whose stripes ye were healed. 25 For ye were as sheep going astray; but are now returned unto the Shepherd and Bishop of your souls.
Ephesians 4:30 And grieve not the holy Spirit of God, whereby ye are sealed unto the day of redemption. 31 Let all bitterness, and wrath, and anger, and clamour, and evil speaking, be put away from you, with all malice: 32 And be ye kind one to another, tenderhearted, forgiving one another, even as God for Christ's sake hath forgiven you.
By being left behind, eventually they will forgive others because then they will realize what Christ has done to forgive them and the ones they had grudges against to not do that again.