What does the Bible mean by the mystery of union with Christ?

In biblical terms, the word 'mystery' signifies truths that cannot be deciphered by human reason alone but must be disclosed by God. In Ephesians 5:32, Paul refers to the union between Christ and the church as a great mystery, meaning it operates on a spiritual level that the unaided mind cannot fully comprehend. Throughout Scripture, this idea is reinforced as Paul uses the term 'mystery' multiple times to express truths that are revealed only to those whom God has chosen to enlighten. Thus, while believers may not understand every aspect of this mystery, they are called to accept and rejoice in the divine revelation of their union with Christ.