Be eager and strive earnestly to guard and keep the harmony and oneness of [and produced by] the Spirit in the binding power of peace. 4 [There is] one body and one Spirit--just as there is also one hope [that belongs] to the calling you received—5 [There is] one Lord, one faith, one baptism, 6 one God and Father of [us] all, Who is above all [Sovereign over all], pervading all and [living] in [us] all. 7 Yet grace (God's unmerited favor) was given to each of us individually [not indiscriminately, but in different ways] in proportion to the measure of Christ's [rich and bounteous] gift. Ephesians 4:3-7 AMP
Jesus died to bring peace between God and man. The word peace denotes wholeness; unbroken fellowship; meaning nothing missing, nothing broken. Jesus died to bring God and man together into harmony, unity, and oneness. You have to have peace to have harmony, harmony to have unity, and unity to achieve oneness. Peace has binding power—it both initiates and maintains harmony between two or more people or beings.
According to the American Heritage Dictionary of the English Language, the word harmony means agreement in feeling or opinion; accord; a pleasing combination of elements in a whole; a combination of sounds considered pleasing to the ear; a coalition of parallel passages.
When we sing together, we sing the melody and the harmony. The harmony comprises different notes than the melody, but they complement and flow with the melody and make the whole song sound richer and fuller. If the harmony is off and clashes with the melody, then it cannot be called the "harmony" any more, so the very word itself tells us that, although there is a difference between the two, they flow together in unison and oneness.
There is a oneness between the Godhead (Father, Son and Holy Spirit)—they cannot be divided or separated—and yet they have very different functions. This is God's plan for the Body of Christ. God created us each very differently, with different functions and different ways of operating, but with the intention that we would flow together, help and complement one another.
And His gifts were [varied; He Himself appointed and gave men to us] some to be apostles (special messengers), some prophets (inspired preachers and expounders), some evangelists (preachers of the Gospel, traveling missionaries), some pastors (shepherds of His flock) and teachers. Ephesians 4:11 AMP
In order to create and preserve unity, we need to be comfortable with who we are in Christ, with our gifts, with our anointing, and with what He has called us personally to do. Also, if we want to walk in peace and accomplish God's purpose for our lives individually and for the earth as a whole, it is crucial that we recognize, respect, appreciate, and receive one another's differences.
His intention was the perfecting and the full equipping of the saints (His consecrated people), [that they should do] the work of ministering toward building up Christ's body (the church), 13[That it might develop] until we all attain oneness in the faith and in the comprehension of the [full and accurate] knowledge of the Son of God, that [we might arrive] at really mature manhood (the completeness of personality which is nothing less than the standard height of Christ's own perfection), the measure of the stature of the fullness of the Christ and the completeness found in Him. Ephesians 4:12-13 AMP
The Lord gives the gifts of the Spirit and the offices of the five-fold ministry to the Body of Christ to perfect and equip the Body for the work of the ministry and to build each other up, so that we become unified—attain oneness—in our faith and so that, in the process, we would grow up into maturity and become perfected and completed, meaning that we become more and more like Jesus, Himself!