I have other sheep that do not belong to this fold. These also I must lead, and they will hear my voice, and there will be one flock, one shepherd.
These other sheep, Jesus says today, he must lead to his kingdom. Jesus does this through his Church. God the Father wants to bring all people into his kingdom, and Jesus died and rose to restore our broken relationship with the Father. In his lifetime, Jesus never wandered beyond a very small area around the Dead Sea and the Sea of Galilee in modern day Palestine. How did he intend to draw these others into his one flock if he never actually travelled beyond his very narrow geographical boundaries? He entrusted this mission to his Church. In his time on earth he couldn't travel personally to everyone he wanted as part of his flock (especially considering he wanted, or rather wants, you, and you weren't born yet). He instituted his Church, invested her with his authority, and sent her to "make disciples of all nations, baptizing them in the name of the Father, the Son, and the Holy Spirit."
Christianity today is not doing a great job of representing the one flock under the one shepherd that Jesus intended. How many Christians denominations are there, and how often do their beliefs flatly contradict each other? How often do congregations split over differences in belief? Surely this cannot be the one flock that Jesus talked about. It looks more like several flocks that bicker with each other with one shepherd trying to bring them back together. The sad divisions in Christianity hurt our witness in the world. As we work and pray for visible unity (the assumption: that we are working and praying for visible unity), it is important that we not ignore our differences in the name of unity, but rather we must focus equally on what beliefs we hold in common and what beliefs continue to separate us.
N.B. One of the most fruitful areas of dialogue have been the Catholic and Lutheran dialogue. I recommend reading the Joint Declaration on Justification, released several years ago.