The King forgives a a servant the debt they owed him. That servant goes on to proves unwilling to forgive someone who owed him a debt. When the King heard of this, he was very upset and scolded him and then comes the last two verses
"And his lord was wroth, and delivered him to the tormentors, till he should pay all that was due unto him. So likewise shall my heavenly Father do also unto you, if ye from your hearts forgive not every one his brother their trespasses."Now, back when I was just arguing for Eternal Security, this would have been an issue to me. I can see those teaching Salvation can be lost using part of this story's ending out of context. But what I emphasized in Bold is the key, it was clearly not an eternal punishment.
The fact that I've shown Aionios doesn't always mean Eternal. Means we can clearly apply this to The Lake of Fire.
And before anyone tries to argue this is only for Believers who fail to forgive. The tone of this story is clearly that this person being someone who the Lord had already forgiven shown mercy on, made him more angry. So it seems illogical to suggest a non Believer's punishment for their debt would be worse.