Colossians 1:15-17 establishes that Christ is infinite (beginning and end) and sovereign (absolute rule over all). You might say that verse 18 makes a transition from the natural to the spiritual by saying "He is also the head of the body, the church." The body is Paul's metaphor saying that as the brain directs and controls the functions of the body so Christ has authority over the operation of his church. The Greek word "ekklesia" defines the church as his called out ones. "You have come to Mount Zion . . . to the general assembly and church of the first-born who are enrolled in heaven . . . and to the spirits of righteous men made perfect" (Heb 12:22-23). Paul explained to the Ephesians that they were initially "separate from Christ, excluded from the commonwealth of Israel, and strangers to the covenant of promise" (Eph 2:12). God had originally chosen Israel saying, "'You are a holy people to the Lord your God; the Lord your God has chosen you to be a people for His own possession out of all the peoples who are on the face of the earth'" (Dt 7:6). But Israel decided not to follow God. However, "as far as election is concerned, they are loved on account of the patriarchs, for God's gifts and his call are irrevocable" (Ro 11:28-29). Consequently, Christ turned to the Gentiles and "is the mediator of a new covenant, that those who are called may receive the promised eternal life" (Heb 9:15). Peter cites that the Gentiles therefore became "a chosen people, a royal priesthood, a holy nation, a people belonging to God" (1Pe 2:9). Paul explained that "in Him also we have obtained an inheritance, having been predestined according to His purpose who works all things after the counsel of His will" (Eph 1:11). You are "called according to His purpose" (Ro 8:28). He "has saved us and called us with a holy calling, not according to our works, but according to His own purpose and grace" (2Ti 1:9).
"He is also head of the body, the church" (Col 1:18), "He Himself being the Savior of the body" (Eph 5:23). He has the supremecy because its members owe allegiance since he has redeemed them. "The church is subject to Christ" (:24). God "put all things in subjection under His feet, and gave Him as head over all things to the church" (:22). This is reflected in the wording of Colossians 1:15-20 and Ephesians 1:21-23: "all things"(6); "He is"(4); "through Him"(3); "in Him"(2); "all the fullness"(2); "all creation", "all rule", "His", "He Himself", "by Him", "for Him" (1). "Christ is the head of every man, and the man is the head of the woman, and God is the head of Christ" (1Co 11:3). The church is the precursor of Christ's ultimate kingdom. It is "that in Himself He might make the two into one new man" (Eph 2:15) to "reconcile them both in one body to God through the cross" (:16). We become "fellow-citizens with the saints, and are of God's household" (:19) "in whom the whole building, being fitted together is growing into a holy temple in the Lord" (:21).
God "calls you into His kingdom and glory" (1Th 2:12) and it is he "through whom you were called into fellowship with his Son" (1Co 1:9). Paul told them that God has "called you by the grace of Christ" (Gal 1:6) and had "set me apart from birth and called me through His grace" (:15). "Whom He predestined, these He also called" (Ro 8:30) and "chose us in Him before the foundation of the world" (Eph 1:4). You are "those who have been chosen of God" (Col 3:12). "There is one body and one Spirit-just as you were called to one hope whom you were called" (Eph 4:1). "From the beginning God chose you to be saved through the sanctifying work of the Spirit" (2Th 2:13). He chose you "through belief in the truth" (:13) and "called you to this through our gospel" (:14). There are actually two terms here of "calling" and "choosing" as Peter advises to "be all the more diligent to make certain about His calling and choosing you" (2Pe 1:10). "From the standpoint of the gospel [calling] they are enemies . . . but from the standpoint of God's choice [choosing] they are beloved" (Ro 11:28). "In Him, you also, after listening to the message of truth, the gospel of your salvation-having also believed, you were sealed in Him with the Holy Spirit of promise" (Eph 1:13). Paul said, "I entreat you to walk in a manner worthy of the calling" (Eph 4:1) of God "who has saved us, and called us with a holy calling" (2Ti 1:9). "As long as you practice these things, you will never stumble" (2Pe 1:10) if you "let the peace of Christ rule in your hearts, to which you were called in one body" (Col 3:15). "In this way the entrance into the eternal kingdom of our Lord and Savior Jesus Christ will be abundantly supplied to you" (2Pe 1:11).