When does God credit His righteousness to a person?
God imputes His righteousness when a person believes the gospel: that Christ died, was buried, shed His blood, and rose from the dead. As God saw Abraham believe and counted him in right standing, so He credits His righteousness to the believer. It is all of His righteousness, none of our works.
Les explains this is about salvation, not Christian experience or Christian works. The one who works tries to merit favor with God and put God in debt, but God will never owe any man anything, not even a righteous man like Abraham. All are sinners by virtue of being born from Adam, and the only remedy for sin is the gospel: Christ died, was buried, shed His blood, and rose from the dead. When a person believes it, just as God saw Abraham believe, God imputes to us His righteousness. It is none of ours; it is all of His.