The Lord will unite us; we must respond to Him.
I pray not only for [the Apostles], but also for those who will believe in me through their word, so that they may all be one, as you, Father, are in me and I in you, that they also may be in us, that the world may believe that you sent me. (John 17:21)
He will turn the hearts of Christians towards each other:
Now I am sending to you Elijah the prophet, before the day of the Lord comes, the great and terrible day; he will turn the heart of fathers to their sons, and the heart of sons to their fathers, lest I come and strike the land with utter destruction. (Mal 3:23-24)