How do we know that we are part of the bride of Christ?

Believers are part of the bride of Christ because of the sovereign grace of God that selects a people for Himself. This selection is unmerited and entirely based on God's grace, as seen in scriptures such as Ephesians 1:4-5, which states that we are chosen in Christ before the foundation of the world. Furthermore, it is the work of the Holy Spirit that regenerates our hearts, enabling us to see and appreciate the beauty of Christ. When we believe in Him and are washed in His blood, we become part of His beloved bride.