Considering we have so many denominations now, isn't Christian unity an impossibility?
This question is an ancient, and difficult, one. Before we can project how Christian unity might take shape in the future, we should look at how Christians have dealt with unity, schisms, and ecumenism in the past.
In A.D. 402, Augustine pled with the breakaway Donatists: "Why have you severed yourselves, by the heinous impiety of schism, from the unity of the whole world?" He challenged their disdain of the catholic, or universal, church with an accusation of his own. "But you, by charging the good wheat [the Catholic church] with being tares, have proved yourself to be tares; and what is worse, you have prematurely separated yourselves from the wheat." He enjoined them to return to the Church: "Awake to the interest of your salvation! Love peace, and return to unity!"
The Donatists ignored Augustine's plea, and his call to unity has gone unheeded for centuries. Church councils sought to reconcile factions to each other, by arriving at an orthodoxy consistent with Scripture and satisfactory to most, if not all. Instead, divisions deepened, beginning with the Council of Nicea, which drove a wedge between Constantine's Church and the "heretical" Arians. Later, Monophysites from Egypt, Nestorians from Syria, and, in Augustine's time, Donatists from Carthage, broke away from the Catholic church, as these new "sects" claimed their own turf.
Unfortunately, Christians have divided along territorial lines ever since. In 865, Pope Nicholas's claim to authority "over all the earth, that is, over every church," didn't sit well with the patriarch in Constantinople. Eastern and Western attempts to foster cooperation and understanding suffered another ...