Why can't God exact justice twice?

The assertion that God cannot exact justice twice is foundational to understanding the grace of God in salvation. When Christ died on the cross, He paid the penalty for the sins of His people fully and finally, as reflected in Romans 6:23 where the wage of sin is death. Therefore, it would be unjust for God to penalize again those for whom Christ died since the penalty has already been satisfied. This doctrine reassures believers that their salvation is secure and that they have been made perfectly righteous through Christ's sacrifice, ensuring their eternal security in Him.