How do we know Christ's righteousness is credited to believers?

The union of believers with Christ is a central theme in Reformed theology. Romans 5:19 specifies that 'for as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous.' This highlights that through Christ's obedience, believers receive His perfect righteousness. Much like Jonathan’s armor bearer who followed his lead up the cliffs, believers are credited with Christ's righteousness because they are united with Him in His work, symbolizing true transformation and acceptance before God.