There are many challenges to unity. One of the most often used is "Do not be unequally yoked with unbelievers." (2 Cor.6:14. This means that one has judged self to be believer, a true believer and the other is not worthy of being called a believer regardless of any confession.
Psalms 133 indicates when we dwell together in unity it is there that the Lord commands a blessing, even life evermore. This would be another blessing beyond the John 17 prayer of Jesus which He asks the Father to make us one in order that the World may know.