I've mentioned that view of mine before, why do I believe that?
For starters the term is only used in Parables. You have to be careful building Doctrine off Parables, things aren't always what they seem.
First of all, to fellow believers in Eternal Security (True Eternal Security, without any Calvinist or Repentance based qualifiers) who are uncomfortable with this suggestion for whatever reason, read Matthew carefully. There is no way to get away with saying the people Jesus describes being sent there were never saved. In 22:13, 24:51 and 25:30 they are servants, the unsaved are never servants of God, they're in rebellion against him. The people sent there are contrasted not with all believers but those who receive rewards based on their works.
And in 8:12 they're the "children of the kingdom". Most people want to in this context make that phrase refer to Israel. But it's only other usage in Matthew 13 clearly identifies them as the Saved.
Now the frequently linked "Wailing and Gnashing of Teeth" is an idiom of grief and sorrow. That is sometimes likened to Judgment of the Unsaved (I never think it's actually Hell directly), like in it's one appearance in Mark. But it certainly is not required to always be in that context.
Jude 13 also refers to the people being in the darkness of blackness forever but not being tormented.
It can make sense to describe damnation as "Outer Darkness" if you accept the Un-Biblical yet popular idea that Hell or the Lake of Fire is "eternal separation from God". But not if you believe what The Bible says. Revelation 14:10 describes the Lake of Fire as being in God's presence. The fire and brimstone that burns the damned emanates from God's presence in Isaiah 30:33 and II Thessalonians 1:8-9. God Himself is called a consuming fire in Deuteronomy 4:24 and Hebrews 12:29. So it's neither "outer" nor "darkness".
New Jerusalem is defined as where the Light is in Revelation 21:23-25. Then verse 26 contradicts those who insist all the Saved are the Lambs Wife, it says there are Nations of the Saved outside New Jerusalem. The people listed here though still get to come to New Jerusalem to worship it seems.
Revelation 22:5 returns to discussing how New Jerusalem is where the Light is.
People who are Universalists of some kind like to point to Revelation 22:15, where it seems to them like the same people sent into the Lake of Fire earlier are now merely outside The City. And you can't argue the Lake of Fire is outside the City because I already showed it is The Presence of God. It isn't clear these are the same people however.
People who teach Salvation can be lost (or the fifth point of Calvinism/false repentance doctrine) love to use 1 Corinthians 6:9-10, Galatians 5:21 and Ephesians 5:5. All these are vice lists warning that those who continue in those sins will not Inherit the Kingdom, and they're clearly warnings to already Saved Believers. But they do not say salvation can be lost, only inheritance. Like the Prodigal Son a Believer can squander his inheritance but he'll never lose his Sonship. They are the believers not allowed to enter New Jerusalem.
The Bema Judgment will not be a Judgment where anyone is really scolded or punished for their Sins. It is a handing out of Rewards only. But one of those Rewards is for keeping your Flesh under control, the Imperishable or Incorruptible Crown.
Luke 8's account of the Parable of the Sower divides people who hear the word into four groups in verses 12-15. Only the first, those who never believed, are described as unsaved. There are people who will believe for a little while and fall way, I discussed Hebrews 6 elsewhere. Also there are those that are believers till the end but are also very worldly. And then those that are fruitful. Only the Fruitful I think will really be New Jerusalem.