The translation of the Greek pleroma, which can be either active or passive—either ‘that which fills’ or ‘that which is filled up’, which is a complication affecting the meaning of passages in the NT. It is used in Mark 6: 43 of the broken pieces of bread which filled twelve baskets, but in John 1: 16, ‘we have all received from his fullness’, is explained by 1: 14, where the Word is filled by grace and truth.
The word pleroma is found in the LXX but in NT times the Gnostics used it for the totality of all reality and this may be why it is used in Eph. 1: 23 and 3: 19. The fullness of God is in Christ; but these verses are unclear whether the meaning is also that the Church is filled by Christ. Perhaps the meaning is that Christ (head of the Church) and the Church (the Body of Christ) are a unity; both are filled by God, who fills all (is supreme).