Will mankind on earth reattain the original state of Adam and Eve?
The biblical answer is YES!. St.Paul even boldly exclaimed it.
"For through the law I died to the law so that I might live to God. I have been crucified with Christ, and I no longer live, but Christ lives in me. The life I live in the body, I live by faith in the Son of God, who loved me and gave Himself up for me. 21I do not set aside the grace of God. For if righteousness comes through the Law, Christ died for nothing.”" (Galatians2:19-21)
Jesus Christ is the New Adam, having a sinless body and free from concupiscence of the flesh and not only have Divine Life but Jesus Christ is the Life.
John 14:6 New International Version (NIV)
6 Jesus answered, “I am the way and the truth and the life. No one comes to the Father except through me.
St.Paul boldly exclaimed that he had reach Theosis or Divinization, the Spirit of Christ fully and perfectly dwells in his being. Therefore, St.Paul had received the promised of Jesus Christ that He will make all things new, St.Paul is a new creation cleanse from original sin and wash away even the stain of sins or concupiscence.
So, the Priest was right in your posted statement.
"Now, I've heard a priest say that God through Christ's death on the cross all of humanity will go back to the original state. (your statement)
The blood of Jesus Christ washes away our original sin, and living a Eucharistic Life like what St.Paul had lived is the key in overcoming the concupiscence. And we know the Jesus Christ became the Wisdom of God.
It is because of him that you are in Christ Jesus, who has become for us wisdom from God—that is, our righteousness, holiness and redemption. (!Corinthian1:30)
And the Spirit of Wisdom or the Spirit of Christ will not enter a soul that is defiled.
Wisdom 1:4 Douay-Rheims 1899 American Edition (DRA)
4 For wisdom will not enter into a malicious soul, nor dwell in a body subject to sins.
Is St.Paul the only one who became a new creation in the history of the Catholic Church?
The answer is NO, there are so many incorruptible saints and so many mystics of the Church who lived a Eucharistic Life with no other sustenance other than the body & blood, soul and divinity of Christ.
See this link on the mystic of the Church and the Incorruptible Saints who became the New Adam having a perfect Divine Life and holy & blameless before God.
MYSTICS OF THE CHURCH
THIS WEBSITE IS DEVOTED TO THE EXTRAORDINARY MYSTICS AND VISIONARIES OF THE CHURCH, ESPECIALLY THOSE WHO ARE LESSER KNOWN, SUCH AS ST GEMMA GALGANI, BLESSED ALEXANDRINA DA COSTA, SR CONSOLATA BETRONE, THERESE NEUMANN, REV. PERE LAMY, GABRIELLE BOSSIS, JOSEFA MENENDEZ, MARTHE ROBIN, SERVANT OF GOD LOUISE LATEAU, BLESSED ANNA MARIA TAIGI, SISTER MARY OF THE HOLY TRINITY, SISTER MARIA ANTONIA AND OTHERS. THE AUTHOR ENDEAVORS ALWAYS TO BE IN COMMUNION WITH THE CATHOLIC CHURCH AND ITS TEACHINGS.
With regards to the Incorruptible Saints the most famous and the recent are St.Padre Pio and St.John Paul II the Great, etc. These incorruptible saints are the like New Adam by perfectly bearing the image of Christ in their pious life.
My question is: Will this happen on Earth, or is it meant for after the Second Coming and Resurrection? What Catholic teaching is the priest referring to?
Yes, we already a witnessed to all the mystics of the Church and the incorruptible saints. And the Catholic teachings is written in CCC1000, it means by living a Eucharistic Life our corrupted body will be transform into a transfigured body as the body.blood, soul & divinity of Christ will fully and perfectly dwells in us if we live a pious life like St.Paul, St.Padre Pio, St.John Paul Ii, etc.
1000 This "how" exceeds our imagination and understanding; it is accessible only to faith. Yet our participation in the Eucharist already gives us a foretaste of Christ's transfiguration of our bodies:
Just as bread that comes from the earth, after God's blessing has been invoked upon it, is no longer ordinary bread, but Eucharist, formed of two things, the one earthly and the other heavenly: so too our bodies, which partake of the Eucharist, are no longer corruptible, but possess the hope of resurrection.556