1 CORINTHIANS 6:17
To the Ephesians, Paul compares this spiritual union to the union that exists between a man and a woman when they are married: “And the two shall become one flesh. This is a great mystery, but I speak concerning Christ and the [Ekklesia]” (Ephesians 5:31,32). Certainly this is a great mystery.
How God is able to make us one spirit with Jesus is beyond human knowledge. But this we know: however it is accomplished, it has its beginning in the Cross. The Cross is the starting point of our union with Christ. In the Cross, God sees us in the place of Christ and sees Christ in the place of us. That is to say, in the Cross, all our sinfulness is attributed to Christ, and all His righteousness is attributed to us. How wonderful for us – but how terrible for Him!