How do we know union with Christ is true?

We know that union with Christ is true because it is grounded in the sovereign will of God. As stated in 1 Corinthians 1:30, we are in Christ because of God's purpose and choice, highlighting His sovereignty over salvation. This union is validated through genuine faith in Christ, which results in an inner transformation—a new creation as described in Galatians 6:15. The evidence of this transformation is the work of the Holy Spirit in our lives, making us aware of our sinfulness, our need for Christ, and producing in us a love for Him and His work. Importantly, true faith rests not on our efforts but solely on God's grace, thus aligning with the doctrine of sovereign grace that emphasizes reliance on God's work in our salvation.