What does the Bible mean by 'federal representative union'?

The doctrine of 'federal representative union' refers to the concept that believers are represented in Christ, participating in everything He accomplished. Just as humanity sinned through Adam, was represented in his disobedience, so too do believers participate in the righteousness of Christ through faith. Romans 5:18 explains that just as the offense of one man brought condemnation to all, the righteousness of one, Jesus Christ, brings justification and life to all who believe. This union underscores the covenantal nature of salvation, highlighting that our standing before God is secured through Christ’s sinless life, death, and resurrection.