Union with Christ is the spiritual and mystical work of God's grace by which believers are inseparably joined to Jesus Christ as their head. Through this union, the elect participate in his life, death, and resurrection, enjoying communion with him and one another.
Union with Christ is the foundational blessing of the gospel. As noted in the 1689 LBCF Ch.27 §1, all saints are united to Christ, their head, through the indwelling of his Spirit and the gift of faith. While Reformed and Presbyterian traditions differ on various aspects of covenantal application, there is broad agreement that this union is a work of God's grace, described as both spiritual and mystical (Westminster Larger Catechism Q.66). It is not a merger of essence, but a real, life-giving bond where the believer is joined to the Savior.
Because we are united to Christ, his story becomes our story. We share in his graces, his death, and his glorious resurrection (1689 LBCF Ch.27 §1). As the Apostle Paul writes, 'I have been crucified with Christ; and it is no longer I that live, but Christ living in me: and that life which I now live in the flesh I live in faith, the faith which is in the Son of God, who loved me, and gave himself up for me' (Galatians 2:20). This means we are no longer defined by our sin, but by his righteousness. We are more flawed than we dared believe, yet more loved in Christ than we dared hope.