How do we know the mystery of Christ and the Church is true?

The mystery that Paul speaks of is made manifest through the revelation given by the Holy Spirit. As indicated in 1 Corinthians 2:7-10, the deep truths of God are revealed to us by His Spirit. Believers are granted understanding of this union that is otherwise hidden from the world. The profound nature of this relationship is found throughout Scripture, demonstrating that just as a man and woman are joined in marriage, believers are eternally united with Christ. Ephesians 1:22-23 solidifies this by highlighting that Christ has been given as the head over all things to the Church, establishing its importance and the reality of this union.