Why does God declare a believing sinner just?
God declares a believing sinner just because that person believes the gospel, and justification is His judicial act as Judge. Looking from the bench, He declares the guilty sinner to be just as if he had never sinned. This justification is freely by His grace, not by any human effort or cost.
Les teaches that justification is a judicial act of God, pictured in Romans as a courtroom scene where God as Judge builds His case against humanity. From His position on the bench, He looks down at helpless sinners and says, because you believe the gospel, I now declare you just as if you have never sinned. Even though the sinner is guilty, God declares him righteous. This justification is free, without cause, without cost, and without a penny involved, flowing from His grace. Les also connects it with forgiveness: God can justify us because we are forgiven, and He can forgive us because we are justified, and it all rolls together in salvation, prompted by the power of God, not by anything man can do.