And we can be very optimistic about reaching this goal [of unity]. Jesus gives another promise related to doing this greater work.
Whatever you ask in my name, this I will do, that the Father may be glorified in the Son. If you ask me anything in my name, I will do it.
Jesus isn't giving a general promise about our prayers. He is talking about this greater work that we are to do. So, overcoming the sometimes large obstacles to loving one another and being of one mind and all the rest is possible because Jesus promises to grant our prayers for success in these things. And I suspect one thing that will happen in response to prayers like this is that Jesus will grant us more of the presence of the Spirit to expose and remove sin and replace it with a holy love that unites.
from Greater Works