You don't know that. And yet we'll all continue to struggle against sin in this life-but now we can also overcome it. Man was not created to be a sinner, after all.
The writer of Hebrews, there in chap 8 and also in chap 10, is quoting Jer 31:
"I will put my laws in their minds
and write them on their hearts.
I will be their God,
and they will be my people.
No longer will they teach their neighbor,
or say to one another, ‘Know the Lord,’
because they will all know me,
from the least of them to the greatest.
For I will forgive their wickedness
and will remember their sins no more.”
By coming to know God we come to believe in, hope in, and, most importantly, to love Him. Beginning with faith, this is what makes us His people, and that vital union, which results in those virtues, is the essence and basis of man's true righteous, a righteousness apart from the law that the Law and the Prophets could only testify to but could never accomplish in us (Rom 3:21). The way to this is reconciliation with God, the heart of the gospel, the object of our faith, the reason Jesus came.
"Apart from Me you can do nothing" (John 15:5).
That's how/why John 17:3 works:
"Now this is eternal life: that they know you, the only true God, and Jesus Christ, whom you have sent."
As we come to know Him personally, as one of His children now, He justifies us, the ungodly; He 'puts His law in our minds and writes it on our hearts'. And the primary name of this righteousness is love, God's own nature, which fulfills the law by its nature (Rom 13:1, Gal 5:14), without even needing to hear the law.
"And hope does not put us to shame, because God’s love has been poured out into our hearts through the Holy Spirit, who has been given to us." Rom 5:5