Because we’re not only forgiven but washed, cleansed, and made new creations enabled by the Holy Spirit to walk as children of God, meaning uprightly, in justice and truth, in obedience of Gods will. But we’re still not forced to obey any more than Adam was at the beginning.
But we’re expected to live this way; the New Covenant was never about relieving man of the obligation to be righteous but rather about finally giving us the true means to that righteousness, in partnership with God.
“Apart from Me you can do nothing”. John 15:5
Adam had thought otherwise. We’re here to learn how wrong Adam was. Man was made for communion with God. Here’s the most important New Covenant prophecy, from Jeremiah 31:33-34 where a new direct and personal knowledge of God, forgiveness of sin, and fulfillment of the law by Him are all part of our coming into this new right standing with Him.
“I will put my law in their minds
and write it 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,”
declares the Lord.
“For I will forgive their wickedness
and will remember their sins no more.”
The definition of that righteousness that He gives us is primarily love, which fulfills the law by its nature (Rom 13:10). But this relationship or communion with God, that justifies, or causes righteousness in us, can be broken, by our walking unjustly, by persistently sinning in some grave or serious manner IOW. Mortal sin is said to oppose love, and destroy love in us. But, with God’s desire for all of us to be with Him, His forgiveness is ever present if we have a true change of heart and wish to return. The sacrament of Reconciliation/Confession serves that purpose, of course. Our wills are never taken out of the equation; the more we will in agreement with Gods will, the more righteous or just we are. That’s what He wants for us and the fulfillment of the Greatest Commandments have the same objective.