Why is the concept of union with Christ important?

The concept of union with Christ is vital in Reformed theology as it signifies that all the benefits of salvation—such as justification and eternal life—are applied to believers through their union with Him. As the sermon illustrates, the number '20' symbolizes completeness, pointing to Christ fulfilling the law not just for Himself but for all of those united to Him. This union grants believers the assurance that, as He is righteous, so are we in this world (1 John 4:17). Therefore, it lays both a foundation for our justification and a call for our lives to honor and reflect Christ's work.