This is the hinge of the entire division, so it deserves a precise answer. The Catholic Church holds that the Bishop of Rome possesses, by Christ’s institution, a primacy not merely of honor but of jurisdiction — a full, supreme, and immediate authority over the whole Church, the East included. That was solemnly defined at the First Vatican Council in 1870.

The Orthodox answer differently, and their case deserves its full weight. For them Rome held a primacy of honor — first among equals, primus inter pares — within a Church governed conciliarly by the five great patriarchates. First, they argue, does not mean over: the head of a council is still bound by the council, and no bishop, Rome included, stands above an ecumenical council. Doctrine was settled at councils by the consensus of the whole episcopate, not handed down from a single see.

The Catholic case is that the first millennium shows the reality of Roman authority long before it was given its 1870 precision. A condemned bishop could appeal to Rome by the canons of Sardica (343), “honour[ing] the memory of Peter the Apostle.” At Chalcedon (451) the Fathers received the doctrinal letter of Pope Leo with the cry, “Peter has spoken through Leo.” To end a schism in 519, the Eastern bishops signed the Formula of Hormisdas, confessing that in the Apostolic See “the Catholic religion has always been kept unsullied.” Catholics read these not as courtesies but as the Petrine office at work.

And here honesty is required, because the evidence is genuinely contested. The official Catholic–Orthodox dialogue itself admits “differences of understanding with regard to the manner in which [primacy] is to be exercised, and also with regard to its scriptural and theological foundations.” And the witnesses themselves carry caveats: the appeals canon of Sardica was the act of a Western synod the East never received as ecumenical, and it granted Rome a retrial by neighboring bishops rather than the right to judge every case himself; the Formula of Hormisdas was signed under imperial pressure, and the East never treated it as a standing charter of papal power. The Orthodox read Rome’s interventions as appellate and moral, not monarchical; Catholics read them as jurisdiction in substance. Both now agree Rome was first; what they still dispute is what that primacy is. That — not the sacraments, not the creed — is the knot on which reunion turns.