There is one body and one Spirit, just as you were called in one hope of your calling.
Ephesians 4:4 (NKJV)
Believers form one body which is the Church of Jesus Christ, although they individually live out their personal relationship with Him. Only those who have placed their faith in the death and resurrection of Jesus for the forgiveness of sins and the assurance of eternal salvation are called to be part of this body. This is why the Bible urges us to be one in spirit, in faith, and even in our hope.
To keep this unity, we cannot do it according to our own moral thoughts nor our personal efforts, we need the Holy Spirit who is the inner force unifying the Body. He lives in each of us, because He is the seal that guarantees our salvation. However, His presence within us cannot keep this unity bound unless we walk in obedience to Him.
As the people of God, we should desire this unity within the Church, for it will be a practical manifestation of our faith and obedience to God. We must support each other, protect each other, and live in perfect harmony so that our faith will be manifest to those who do not know Christ.