The forbearance of God is past because Christ has died and brought righteousness to faith.
Abraham was not justified before God by works; he believed God and faith was counted to him for righteousness. If one works, righteousness is not grace but debt. "But to him that worketh not, but believeth on Him that justifieth the ungodly, his faith is counted for righteousness."
David speaks of the man to whom God imputeth righteousness without works saying, "Blessed are they whose iniquities are forgiven, and whose sins are covered."
Abraham did not receive righteousness by law but before the law that he might be the father of all who believe. God's promise that Abraham "should be the heir of the world, was not... through the law, but through the righteousness of faith."
All is by faith that it might be by grace so that the promise might be sure to all.
Abraham is the father of us all on the basis of our faith. He believed God Who could raise the dead and "calleth those things which be not as though they were." He believed that God could act above nature in giving him a son in his old age.
We, like Abraham, can be strong in faith, giving glory to God. Righteousness is imputed to us also who believe on Him who raised up Jesus our Lord from the dead, "who was delivered for our offenses, and was raised again for our justification."