Wouldn’t it have been understood by all the churches from the 1st Century onward?
After living with Jesus Christ for three years, the Apostles still didn’t understand the simplest of doctrines. It took their experiencing His death and resurrection and then the outpouring of the Holy Spirit at Pentecost, before they finally understood.
So, whether every single Church member perfectly understood the Doctrines of our Lord, I don’t know.
Wouldn’t Christians in the year 100 or 200 have understood that the Bishop of Rome was the leader of all the churches?
Most did. And some wrote about it.
Ignatius of Antioch
You [the See of Rome] have envied no one, but others have you taught. I desire only that what you have enjoined in your instructions may remain in force (Epistle to the Romans 3:1 [A.D. 110]).
But since it would be too long to enumerate in such a volume as this the succession of all the churches, we shall confound all those who, in whatever manner, whether through self-satisfaction or vainglory, or through blindness and wicked opinion, assemble other than where it is proper, by pointing out here the successions of the bishops of the greatest and most ancient church known to all, founded and organized at Rome by the two most glorious apostles. Peter and Paul, that church which has the tradition and the faith which comes down to us after having been announced to men by the apostles. With that church, because of its superior origin, all the churches must agree, that is, all the faithful in the whole world, and it is in her that the faithful everywhere have maintained the apostolic tradition (Against Heresies 3:3:2 [A.D. 189]).
More importantly, wouldn’t the Bishops and elders in the Eastern Churches have understood it as well and not resisted when Rome claimed the “authority of the keys” in 250?
Most did. And most remained united to the Pope until the 10th century.
Which is the first time on record that the Bishop of Rome asserted Matthew 16 as scripture to support the primacy of Rome.
I don’t know. But Clement of Rome, the Pope after Peter, was already exerting that authority in the first century:
Clement of Rome
Accept our counsel and you will have nothing to regret. . . . If anyone disobeys the things which have been said by him [Jesus] through us, let them know that they will involve themselves in no small danger. We, however, shall be innocent of this sin and will pray with entreaty and supplication that the Creator of all may keep unharmed the number of his elect (Letter to the Corinthians 58:2, 59:1[A.D. 95]).
This wasn’t a complex theological issue. It was about authority.
True. But neither was it complex to believe that God was the King of Israel. Yet they rebelled and wanted a human King. Before that, the Hebrews could see God leading them through the desert and they still rebelled.
I think you’ve neglected to factor in, the fallen human nature.