Antonius,
Any bishop who teaches that which can not be demonstrated in the Holy Scriptures or the Apostle's teaching, is giving his own opinion and indeed, it can be heretical.
Since there was heresy being taught in the church during the time of John, it is not surprising it has existed ever since.
"while accepting and lauding strange and novel doctrines that erupted 1,500 years after the Apostles.'"
And here you are making a straw man argument - a trope - that is false in its assumption that salvation by grace through faith was never present before 1,500.
It is easily shown that Rome and her peeps and popes hunted down and slaughtered the Waldensians who lived peacefully in the mountains of Italy, long before Luther came on the scene.
This is just one example that disproves your claim FRiend. It is sufficient to do so.
Here are early drawings, showing the slaughter of innocent women and children believers who entrusted themselves to Christ along.
Rome couldn't abide by that.
If you'd like more pictures, there are many that show this peaceful group of believers (who began in the 12th century) taught almost identically what Luther taught hundreds of year later.
It is worth remembering these people of faith, children of God, who were slaughtered by pagan Rome's pope and peeps.
In case you doubt Rome did this, here is the apology Pope Francis issued for the sins of Rome's slaughter of the Waldensians.
You only need to go back step by step and see who Rome slaughtered for teaching truth.