This is not the only reason but it bears mentioning.
Jesus prayed to his Father just before he was to be crucified and the prayer is recorded for us in John 17:20-23.
My prayer is not for them alone.
I pray also for those who will believe in me through their message,
That would include Christians today.
that all of them may be one,
Christians today are not one and we evangelicals are the worst. We will answer for this come judgement day.
Father, just as you are in me and I am in you.
That is quite the high bar for Christian unity.
May they also be in us
Quite the high bar for the Christian walk.
so that the world may believe that you have sent me.
This is key! Jesus wants us to be unified so the world will know that the Father sent the Son. If Jesus says unity causes belief, does it stand to reason that disunity causes dis-belief?
I have given them the glory that you gave me, that they may be one as we are one, I in them and you in me, so that they may be brought to complete unity.
It sounds like Jesus has already given us the glory that the Father gave to him and that we have the ability to do this
Then the world will know that you sent me and have loved them even as you have loved me.
Jesus says again that the unity of his followers will convince the world that he was sent from the Father. He must really mean it.