The Pope does NOT have Papal supremacy over Christians, he has Papal supremacy over Catholics. Big, big difference.
If the Pope has papal supremacy of the Christian churches, do protestant and luciferian churches really exist anymore? How can they if the head of the Christian Church is Catholic?
Wikipedia - Papal supremacy is the doctrine of the Roman Catholic Church that the pope, by reason of his office as Vicar of Christ and as pastor of the entire Christian Church, has full, supreme, and universal power over the whole Church, a power which he can always exercise unhindered: That, in brief, "the Pope enjoys, by divine institution, supreme, full, immediate, and universal power in the care of souls."
The Pope is the Head of the Roman Catholic Church, the worlds most populous religion, and the largest of the 40,000 or so separate denominations of Christianity. The rest are Protestants, most of whom do not recognize Papal authority. About 1/5 of the human race follows some brand of Christ worship.