1) The church is a body of people who have received redemption from creation's fall so that the Father would put Jesus' preeminence on display (Col. 1:14-18)
2) Redemption is for the glory of God: The way in which the God delights in and glorifies each Trinitarian member as it pertains to creation/redemption is by the Father electing some and giving them to His Son; the Son laying down His life so that He may save a people for God's own possession who reflect Him; and the Spirit taking the work of Christ and applying it to the elect (Jn. 6:37-65; Eph. 1:3-14; Col. 1:15-22). According to the eternal covenant, the Father and Son (and, by implication, the Spirit) delighted to do this for their name's sake (Isa. 42:1-8; 49:1-6).
3) Redemption involves restoring image-bearers back into their intended state and condition. The definition of a human being is an image-bearer - particularly, God's image-bearer (Gen. 1:26-27). That image was spoiled by the fall (but not completely destroyed). Redemption involves restoring that image-bearer back into his intended state of fellowship and peace with God and imaging His glory. Because Jesus is the image of the invisible God (Col. 1:15), to image God is to image Jesus.
4) Consequently, God's purpose for His church is to conform them to the image of His Son: "For those whom he foreknew he also predestined to be conformed to the image of his Son, in order that he might be the firstborn among many brothers" (Rom. 8:29). Our gathering as a body is toward this purpose: "But grace was given to each one of us according to the measure of Christ's gift...to equip the saints for the work of ministry, for building up the body of Christ, until we all attain to the unity of the faith and of the knowledge of the Son of God, to mature manhood, to the measure of the stature of the fullness of Christ" (Eph. 4:7, 12-13). This is why we are given spiritual gifts.
5) God commands us to joyfully minister to one another (using the grace/spiritual gifts he has given us) for the purpose of building one another up into Christ's likeness/image (All of Paul's epistles; 1 Pet. 4:9-11).
6) The God-ordained means for doing number 5 is in a local church where all the spiritual gifts come together in unity, and under the authority of the church offices God has established (elders/pastors, deacons) (Eph. 4; 1 Tim. 3:1-15; Tit. 1:5ff). Submitting to and obeying the leaders God has placed in our lives involves coming under their shepherding care as those who have watch over our souls and will have to give an account (Heb. 13:17).
7) A believer is someone who loves his brothers by laying down his life for his brother in service to him. This is not a first-class Christian; this is a Christian. We do not love merely in word and in tongue, but in deed and in truth (1 Jn. 3:10-18).