When we think of Jesus in the garden we think of Him praying for His Father to take this cup from Him. We think of the disciples falling asleep even as Jesus tells them to stay awake and pray that they won't enter into temptation. But in John 17 we see that Jesus also prayed about some other things that were heavy on His heart.
In the first verses of chapter 17 Jesus prays that God the Father would glorify the Son that all the world would be able to have eternal life through His sacrifice. Beginning in verse 6 He prays for "the people whom you gave me out of the world", meaning His disciples. He prays that God would "sanctify them in the truth" and remind them of the words of truth Jesus spoke to them.
Then in verse 20 Jesus prays, "I do not ask for these only, but also for those who will believe in me through their word". He prays for us who believe in Him today! And what is it that He prays for? He prays that we "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". (vs. 21)
Just like peace, Jesus doesn't pray that we are one in the way the world might see it, but instead in the way God intended for unity to come, through faith in His Son. Jesus had perfect unity with His Father. He tells how He stays unified to His Father throughout John, but especially in chapter 15. It goes back to abiding in Him.
In the books on marriage I have read and in my experience I have found something to be true. Marriage is like a triangle. At the beginning there are two people who are at the bottom two corners, but as they both grow toward Christ, the peak of the triangle, they cannot help but grow closer to one another. The same is true of all believers. As we individually grow toward Christ and unity with Him, we will grow in unity toward one another.
And read the end of verse 21 again, "so that the world may believe that you have sent me". Our unity is for our benefit as the body of Christ, but Jesus' prayer is that our unity shows the world something different than the unity the world gives; it shows them that Jesus is truly the Christ. And our unity will show the world that the Father "loved them [the followers of Christ] even as you [the Father] loved me". In other words our unity will attract people to come to Jesus in faith. In our "perfect" unity, the unity that can come ONLY through Christ, the world may see something they can find nowhere else.
And because of that they will come to Jesus.
May Jesus' prayer be answered in us today.