John 17:20,21 ... This is the prayer that Jesus prayed for us: "I do not pray for these alone (His disciples), but also for those who will believe in Me through their word; that they all may be one as You, Father, are in Me and I in You; that they also may BE ONE IN US, that the world may believe that You sent Me."
"That the world may believe You have sent me" ... here Jesus is saying that God the Father is in Jesus and that both of Them are in the believers. In John 14:17, He told His disciples that the Holy Spirit has been with them, but He's going to be in them. So with this, we have all three of Them in us ... God the Father, God the Son, Jesus, and God the Holy Spirit.
1 Corinthians 6:19 ... "Do you not know that your body is the temple of the Holy Spirit Who is in you, Whom you have from God, and you are not your own?" Our body is the temple for God the Father, God the Son, God the Holy Spirit ... and us. We are all in unity.
The church has prayed for years, not realizing that when we become born again, we have that unity. It's there! We are not looking for unity of the churches or the unity of our church doctrines. This "unity" will NEVER happen! Unity is "oneness" ... mankind will never be in complete oneness with one another without God. Unity has already been given to us. Jesus said so in His own prayer in John 17 to the Father.
Reverend Robert W. & Mary C. Butler