God created man and woman to be one flesh, crafting a supernatural unity (Gn 2:24). Adam and Eve sinned against God, and with their fallen nature, humanity has inherited a tendency to fracture into disunity. Before long, mankind was so wicked (Gn 6:5) that God wiped them out in the flood (Gn 7:23).
So God started over again with Noah's family, and again the human race began with unity (Gn 8:16). Before long, man's pride again resulted in a false unity, as men joined to glorify themselves rather than God (Gn 11:4ff). So God had to bring division (cf Lk 12:51), through different languages and physical dispersion.
After many years, God sent His Son "to gather into one all the dispersed children of God" (Jn 11:52). Jesus did this by ascending into heaven and, with the Father, sending forth the Holy Spirit. At Pentecost, the Spirit started over again and reversed man's disunity. The apostles and disciples "were filled with the Holy Spirit. They began to express themselves in foreign tongues" (Acts 2:4). Dispersed people heard and understood the gospel in their own language (Acts 2:11).
Our human nature tends toward disunity. But God makes us "sharers of the divine nature" (2 Pt 1:4) by giving us the Holy Spirit. Only the Spirit can unite us. "Receive the Holy Spirit" (Jn 20:22). Come, Holy Spirit of unity! (Eph 4:3)