The quote of Jesus that there will eventually be one fold and one shepherd is one of the most important in the New Testament. We know from the Lord's prayer that we seek the kingdom of God on earth. This surely aligns with the idea that there will eventually be one fold under one shepherd on earth. The true shepherd must be God, in this case acting through the station of the son, or Christ. But who comprises the "fold" of Christ? The only possible meaning is that the fold are the followers of and believers in Jesus as sent by God as the Christ. But Jesus refers to others not of the same fold which also have to be brought to God. The argument that this means the gentiles rather than the Jews does not stand up to scrutiny. Any person, Roman soldier or otherwise, who believed in Jesus was of his fold. Jesus was not concerned whether they were Jews or not, only that they believed in him. He did not discriminate on the grounds of race or ethnicity.
So who comprises the other fold? This can only mean other people in the world who could not know about Jesus. For practical reasons this comprised most of the people of the world at that time. These also have to eventually be brought to God and included in the one fold. They will not be believers in Jesus as they are of another fold(s), possibly of other religions. If we accept the prophecy, it must lead eventually to one unified godly community on earth, a time of world peace and unity. But how will God achieve this? The revelation of Jesus recorded in the New Testament was completed when he died and ascended. Thus was created the Christian fold comprised of the followers of Jesus ever since. But they are only a minority of the total people on earth, even today. Of necessity there needs be another revelation from God to bring all the people in different folds into one fold, otherwise the prophecy cannot be fulfilled.
We might regard that revelation as the return of Christ on earth, with teachings necessary to create one fold for all. This could only occur when the circumstances were right on earth to enable that revelation to be made known around the world so that all people can have an opportunity to accept or reject it. Given modern technological advances and the rise of the global village, the present is such a time.
So where is that revelation? We need to search for the evidences of teachings from God that have the spiritual power to unite all peoples in one fold. We need to identify the return of Christ with such a message. We know he will come as a thief in the night - the Bible says so many times. So the search will not be easy. It is, however, the most important search of our lives.