Heb 12:1
Therefore we also, since we are surrounded by so great a cloud of witnesses, let us lay aside every weight, and the sin which so easily ensnares us, and let us run with endurance the race that is set before us
See Heb 12:15-17 to understand what sin Heb 12:1 speaks about.
looking carefully lest anyone fall short of the grace of God (unbelief in Jesus which is the sin the world is convicted of, John 16:9); lest any root of bitterness springing up cause trouble, and by this many become defiled; lest there be any fornicator or profane person like Esau, who for one morsel of food sold his birthright. For you know that afterward, when he wanted to inherit the blessing, he was rejected, for he found no place for repentance, though he sought it diligently with tears (no place for repentance is confirmed also in Heb 6:4-6).
And how does one fall short of the grace of God?
See Gal 5:4
You have become estranged from Christ, you who attempt to be justified by law; you have fallen from grace.
So anyone who has "tasted the heavenly gift, and have become partakers of the Holy Spirit, and have tasted the good word of God and the powers of the age to come", (Heb 6:4,5), but then falls away by turning back to the law to determine righteousness, such have fallen from grace and made themselves a transgressor/Sinner.
Gal 2:18
if I build again those things which I destroyed (righteousness by works of the law),I make myself a transgressor/Sinner.
But, I suspect a lot of legalists have never even "tasted the heavenly gift, and have become partakers of the Holy Spirit, and have tasted the good word of God and the powers of the age to come", hence they still have opportunity to repent of their dead works.
The sin the world is convicted of is unbelief in Jesus, John 16:9 (and being under the law is unbelief).
We are to keep His commandment to believe in Jesus, without spot, and blameless.
1Tim 6:12-14
Fight the good fight of faith, lay hold on eternal life, to which you were also called and have confessed the good confession (confess Jesus, Rom 10:9) in the presence of many witnesses. I urge you in the sight of God who gives life to all things, and before Christ Jesus who witnessed the good confession before Pontius Pilate, that you keep this commandment without spot, blameless until our Lord Jesus Christ’s appearing,