Why is a believer's union with Christ important?

A believer's union with Christ is a fundamental principle of the gospel. This union is both legal and spiritual. Legally, believers are justified and declared righteous because of Christ's sacrificial death and resurrection (Romans 5:21). Spiritually, the union is realized when one is born again by the Holy Spirit, moving from death to life (Ephesians 2:1). This relationship means that Christ lives in us and we in Him; thus, our righteousness is not from ourselves but solely attributed to Christ, who fulfills all conditions of salvation on our behalf.