What is the church in relation to Christ?
The church is the body of Christ, and Christ is its head. God gave Him to be head over all things to the church, which is His body. The church is called the fullness of Him who fills all in all, and this fullness can be understood as the complement of Christ.
Les explains from Ephesians 1:20–23 that God raised Christ from the dead, set Him at His own right hand in the heavenlies, far above all principality, power, might, and dominion, and put all things under His feet. He then gave Him to be the head over all things to the church, which is His body. Les points out that the word "fullness" in verse 23 could be translated "complement"—not with an I, but with an E. The Body of Christ, of which believers are members, is the complement of Christ. To illustrate, Les goes back to Genesis 2 and Adam and Eve before the Fall, when Eve was still in Adam.
Which is his body, the fulness of him that filleth all in all.