Response:
Eph 1:23
Which is his body, the fulness of him that filleth all in all.
In Eph 1:23 fullness means: "That which fills up". Literally it means: "The Filing". His body, is filled with Him. He dwells there in all fullness, that is to say Christ in all of His fullness dwells in every believer and therefore is the fullness or the completeness of His whole body which is one with Himself.
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Eph 3:9-19
9
And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ: 10 To the intent that now unto the principalities and powers in heavenly places might be known by the church the manifold wisdom of God, 11 According to the eternal purpose which he purposed in Christ Jesus our Lord: 12 In whom we have boldness and access with confidence by the faith of him. 13 Wherefore I desire that ye faint not at my tribulations for you, which is your glory. 14 ¶ For this cause I bow my knees unto the Father of our Lord Jesus Christ, 15 Of whom the whole family in heaven and earth is named, 16 That he would grant you, according to the riches of his glory, to be strengthened with might by his Spirit in the inner man; 17 That Christ may dwell in your hearts by faith; that ye, being rooted and grounded in love, 18 May be able to comprehend with all saints what is the breadth, and length, and depth, and height; 19 And to know the love of Christ, which passeth knowledge, that ye might be filled with all the fulness of God.Here in Eph 3:9-18, Paul’s desire is that every believer may come to know the mystery of God: that they are the very body of Christ. And that Christ dwells in them in all His fullness and that they all may abound in that fullness in their inner man. That they may grow up into Christ and be filled with the knowledge of Him.
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Eph 4:13
Till we all come in the unity of the faith, and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fullness of Christ:
In Eph 4:13 Paul is saying the same thing that he has said in Ephesians 1:16-19. That Christ might be revealed in every believer. That we all would come to the knowledge of the Son of God. that we would become full grown and complete in the knowledge of Christ. He is saying that the true measure of every believer is Christ Himself and that the work of the Spirit is to bring us to the knowledge of that full measure of Christ Who is in us, that we might know Him in His fullness.