God desires that His Son's followers experience unity. In fact, it was one of Jesus's last prayers for us. However, our idea of unity often bears little, if any, resemble to the Lord's idea of unity.
At the end of his life, Paul was abandoned by almost everyone. Paul confronted Peter—publicly. Jesus Himself called the religious leaders a brood of vipers. How could any of this lead to unity?
God's idea of unity and ours are vastly different. We seek unity based on ethnicity, social standing, denominational distinctives, and so much more—none of which is what Jesus prayed for, gave His life for.
When we refer to doctrinal distinctives, we must be careful not to elevate non-essential issues. While some are important—such as prophecy, church government, and forms of baptism—differences in these doctrines should not cause us to refuse to worship with one another.
Still other doctrines are interesting but should not cause division and disunity. Who was Cain fearful of when he was banished? Who were "the sons of God" mentioned in Genesis 6? Did Christ descend into hell (1 Peter 3:19-20) or does this refer to "Abraham's bosom"? These questions and others like them are interesting to discuss but are not essential to genuine Christian unity.
Instead, we must focus on the Essentials. Among these are the deity of Christ; the substitutionary life, death, and resurrection of Jesus; and salvation by grace through faith alone. There cannot be God-honoring unity with those who do not hold to these and other equally important doctrines.
When Jesus says believer are to be united, it's crucial we understand what He's talking about. We get a glimpse of His perspective of unity in 1 Timothy 1:4-5. Paul tells the young pastor not to pay attention to endless genealogies but to focus on teaching that leads to love from a pure heart, a good conscience, and a sincere faith.
The unity of believers is internal.
Biblical unity doesn't come from us; it's from God. It isn't about trying harder and being nicer. Godly unity is both crushing and costly because it starts with Calvary. Only those who walk in the Spirit, those who have been crushed, will be molded into the image of Christ and experience true unity.
Ephesians 4:3 says we are to be "eager to maintain the unity of the Spirit in the bond of peace."
In John 17:21-22, Jesus prays, "that they [believers] may all be one, just as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me. The glory that you have given me I have given to them, that they may be one even as we are one."
That prayer was answered when the Holy Spirit baptized all believers into the body of Christ. First Corinthians 12:13 says, "For in one Spirit we were all baptized into one body—Jews or Greeks, slaves or free—and all were made to drink of one Spirit."
Though this is truly awesome, there is even more to God-honoring unity. In John 17:23, Jesus says, "I in them and you in me, that they may become perfectly one, so that the world may know that you sent me and loved them even as you loved me." (emphasis mine)
Jesus asks that the believers "may become" perfectly one. Transformation is a process. Sanctification is a visual display of who is, and who is not, a Christian.
Ephesians 4:13 say, "… 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."
The unity of believers is based on our shared life in Christ.
Jesus is praying for the unity of His sheep. John 1:12-13 says, "But to all who did receive him, who believed in his name, he gave the right to become children of God, who were born, not of blood nor of the will of the flesh nor of the will of man, but of God."
In John 17:21, Jesus compares godly unity with the oneness He experienced with His Father. "… that they may all be one, just as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me."
Born again believers share in the divine nature of God, because our unity is like the unity between the Father and the Son. In verse 22, Jesus goes so far as to say, "The glory that you have given me I have given to them, that they may be one even as we are one,"
Bur the Lord doesn't share His glory. So, what does this mean? While there are many perspectives on this, let's look at a few verses to give us an idea.
John 1:14 says, "And the Word became flesh, and dwelt among us, and we saw His glory, glory as of the only begotten from the Father, full of grace and truth."
Verse 16 goes on to say, "For from his fullness we have all received, grace upon grace."
Leon Morris said this about verse 21:
… just as His true glory was to follow the path of lowly service culminating in the cross, so for them the true glory lay in the path of lowly service wherever it might lead them. 
William Barclay said this:
We must never think of our cross as our penalty; we must think of it as our glory … The harder the task we give a student, or a craftsman, or a surgeon, the more we honor him … So, when it gets hard to be a Christian, we must regard it as our glory, as our honor given to us by God. 
In John 17:23, Jesus says, "I in them and you in me, that they may become perfectly one, so that the world may know that you sent me and loved them even as you loved me."
The Holy Spirit comes to dwell within us when we are born again. The world begins to see God's glory at work in and through us as He sanctifies us and develops Christ's character in us.
The fact that the Father loves us as He loves His Son is beyond comprehension—but it's true! That love is our glory, "the common glory" that unites believers and brings us together as brothers and sister in the worldwide family of God.
Unity is not difficult, but it is costly. Our unity must be in Christ alone. Unity begins and is sustained by our union with Jesus. This union brings us together as children of God. There is no vision, organization, plan, dream, or program that could ever cause us to fall irrevocably in love with one another. The only way it can happen is when God transforms our hearts and lives.
If unity is going to take place, then it cannot be based on mere cooperation or group conformity. Any "unity" that is maintained by external pressure rather than the internal work of God's Spirit will fail.
The unity of believers is undefeatable.
Unity that is born of the Spirit, touched with Calvary, descends from heaven, and transforms our hearts is a precious gift. It cannot be defeated, dissolved, or diluted. There is nothing man can do to destroy it. There is nothing the devil can do to destroy it.
As we walk in unity, we will experience the peace and love of Christ despite life's conflicts, hardships, and struggle. It requires effort on our part. We must die to self. We must seek to help others along the journey to godly unity. But, in so doing, unbelievers will see what God means by unity.
In John 17:21, Jesus explains the purpose of unity. "… so that the world may believe that you have sent me."
Verse 23 is similar. "… so that the world may know that you sent me and loved them even as you loved me."
The unity of believers displays the gospel to the world.
In Romans 10:14, Paul writes, "How then will they call on him in whom they have not believed? And how are they to believe in him of whom they have never heard? And how are they to hear without someone preaching? And how are they to preach unless they are sent? As it is written, 'How beautiful are the feet of those who preach the good news!'"
How do we express authentic Christian unity to the world?
The Father loves us as He loves Jesus. In Romans 8:38-39, we catch a glimpse of the vastness of that love. "… neither death nor life, nor angels nor rulers, nor things present nor things to come, nor powers, nor height nor depth, nor anything else in all creation, will be able to separate us from the love of God in Christ Jesus our Lord."
Now, that's love!
A few years ago, I felt as if I had plateaued in my faith. While I knew that wasn't actually possible, it was how I felt. That all changed when my first child was born.
After she was born, she objected to being placed under the warming lamp. But when I walked over to her and spoke to her, she stopped crying, turned to look at me, and smiled.
Although I had been speaking to her long before she was born, when we finally got to meet face to face, when my voice brought her comfort, when she held my finger in her tiny fist, I instantly fell in love. It was different than anything I'd ever experienced up to that point. It was as if a part of my heart that I never knew existed suddenly opened up and began to pour out the overwhelming love a parent has for their child. Soon after, it dawned on me that this was only a fraction of the love God has for us.
In 1 John 4:11, we read, "Beloved, if God so loved us, we also ought to love one another."
It's no wonder Jesus stated that the two greatest commandments are to love God (Matthew 22:37) and to love others (Matthew 22:39).
It's easy for us to love people who are like us, but the love of Jesus stretches across racial, cultural, generational, and economic divides. The love of Jesus unites us. The love of Jesus is a testimony to the world.
As believers, we have different callings on our life and we have different spiritual gifts. But we all have the same purpose: to glorify God in all that we do (1 Corinthians 10:31). We glorify Him by being obedient to His Word and growing in Christ-likeness (John 15:8,10). And we glorify Him as we're molded into the image of Christ (2 Corinthians 3:18).
We also have a common mission. In Philippians 1:27, we are instructed to "… conduct yourselves in a manner worthy of the gospel of Christ, so that whether I come and see you or remain absent, I will hear of you that you are standing firm in one spirit, with one mind striving together for the faith of the gospel."
As we grow to increasingly love the Lord and others, the world will see the unity that Jesus prayed for and the gospel will transform lives.
 The Gospel According to John [Eerdmans], p. 734
 (D. A. Carson, The Farewell Discourse and Final Prayer of Jesus [Baker], p. 198, concurs.