The essence of God is love. God is Love Itself. And this Love being Infinite, has the Power to bring all persons to the state of being animated by divine love and thus suited to the glories of heaven.
The Catholic Encyclopedia mentions a number of saints and Church Fathers who believed in the apokatastasis, the final restoration of all spirits:
newadvent.org/cathen/01599a.htm
To that list, we can add, perhaps at least with respect to humans, Doctor of the Church, St. Therese of Lisieux, if what she wrote in a play is any indication of her thought: (this is from von Balthasar; you can read more in the link)
*Therese has so lively a consciousness of the “always more” aspect of divine mercy that, in a Christmas play for the edification of the members of her religious order, she has various angels assemble around the crib: the “Angel of the Child Jesus” and the “Angel of the Holy Countenance” (the Passion) sing of the infinite love of the Son of Man in anticipation of his coming suffering but also of his Resurrection and triumph. Then there appears the “Angel of the Last Judgment, armed with a sword a pair of scales”. The following excerpt from his lines may be cited here:
“The day of reckoning is coming soon; this impure world will be forced to go through fire. We will see the radiance of his glory, no longer concealed beneath the features of a child; we will extol his triumph and acknowledge him as the Almighty. You will tremble; the inhabitants of the earth will not bear the wrath of this Child, who today is the God of love. He chooses suffering and demands in return only your frail heart. At the time of judgment, you will recognize his power and quake before the avenging God.”
Then “the angel of the Holy Countenance” speaks, requesting of the Child the promised mercy for those sinners whose conversion gives God greater joy than do the ninety-nine righteous who have no need of repentance. After this comes the voice of the Child: “I will listen to your request:
every soul will find forgiveness.” The Angel of Vengeance once again objects: “Do you forget, Jesus that the sinner must be punished; do you forget, in your exceeding love, that the number of the godless is endless? At the time of judgment, I want to punish the crimes, to destroy all the ungrateful; my sword is ready, well will I know how to avenge you!” Then the Child Jesus: "Beautiful angel, lower your sword. It is not for you to judge the nature that I desired to set in being and to redeem. I myself am the Judge of the world, and my name is Jesus. The Angel of Judgment kneels down and, “quite beside himself, wonders at your unutterable love.”*
books.google.com/books?q=%22every+soul+will+find+forgiveness%22&btnG=Search+Books (click on “Dare We Hope…” to read it)
Here’s a non-Catholic resource on universalism:
tentmaker.org/
Some universalist passages in scripture:
1 Cor 15:22
Col 1:20
Rom 5:18
Rom 11:32
I’ll focus on Col 1:20
“and through him
to reconcile to himself all things, whether things on earth or things in heaven, by making peace through his blood, shed on the cross.”
If there are souls or spirits to be forever in hell, then they would not be among the “all things, whether things on earth or things in heaven” that are to be “reconciled” – those in hell are not reconciled! I don’t see any way around this. And note it says Christ came to reconcile all things whether things on earth or things in heaven – or in the heavens – that would include the fallen angels as the heavens (sky or skies) are their abode, even though they are cast out from the beatific presence of God.
In the Old Testament it says that even if a mother should forget her child that God would not forget any of us.