Jesus prays that we would be one as He and the Father are one. Then the world will believe that the Father sent the Son. The people of the world closely observe Christians who are fighting with each other. The devil showcases to non-Christians our racism, denominations, divisions, and jealousies. This turns them off to the gospel. Then, with world evangelization stalled through the disunity of Christians, Satan proceeds to massacre babies in the womb, destroy young people, break up marriages, pervert society, promote his kingdom of darkness, and lead people to eternal damnation. We can see why Jesus has prayed for two-thousand years that Christians would be one.
Jesus' prayer for unity begins to be answered when we are filled with the Holy Spirit. The Spirit is the Source of unity (Eph 4:3). "It was in one Spirit that all of us, whether Jew or Greek, slave or free, were baptized into one body" (1 Cor 12:13). We are "one in the Spirit." The Spirit reverses the divisions which go back to the first sin and were exacerbated at the tower of Babel (see Gn 11:9). By the Spirit of Pentecost, all nations can once again communicate with each other and be restored to the unity of paradise (see Acts 2:5-11). Then, as at the first Christian Pentecost, the Spirit showcases our unity to lead thousands into God's kingdom (see Acts 2:41), and the strongholds of the devil are destroyed (see 2 Cor 10:4). Come, Holy Spirit of unity, evangelization, and victory.