All those Christ lived and died for, they automatically become doers of the law.
You see, God imputed Christ's obedience to the law to them He was made a Surety Heb. 7:22 representative in behalf of.
Righteousness has been charged to their account, and their disobedience imputed to Christ Is. 53:6.
So in God's sight they are righteous, keeping all the law perfectly.
Rom. 5:19
For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous.
Rom. 8:33-34
33 Who shall lay any thing to the charge of God's elect? It is God that justifieth. 34 Who is he that condemneth? It is Christ that died, yea rather, that is risen again, who is even at the right hand of God, who also maketh intercession for us.
But for all the non-elect, whom Christ was not their surety, they must keep all the law perfectly or suffer the second death Rev. 21:8, for the wages of sin is death Rom. 6:23.