There's no truth in the idea that Luther's rejection of the Papacy was a departure from authentic Christianity. Luther was a true Catholic who wanted the visible Church to abandon the false teachings which had taken hold through the autocratic leadership of the popes. He rightly identified the Papacy as the Antichrist, and when he was unable to bring about reform due to the papists holding the reigns of power he was forced into schism. Here's how Luther explained his break from the pope in his Table Talk:
"The chief cause that I fell out with the pope was this: the pope boasted that he was the head of the church, and condemned all that would not be under his power and authority; for he said, although Christ be the Head of the church, yet, notwithstanding, there must be a corporal head of the church upon earth. With this I could have been content, had he but taught the Gospel pure and clear, and not introduced human inventions and lies in its stead. Further, he took upon him power, rule, and authority over the Christian church, and over the Holy Scriptures, the Word of God; no man must presume to expound the Scriptures, but only he, and according to his ridiculous conceits; so that he made himself lord over the church, proclaiming her at the same time a powerful mother, and empress over the Scriptures, to which we must yield and be obedient; this was not to be endured. They who, against God’s Word, boast of the church’s authority, are mere idiots. The pope attributes more power to the church, which is begotten and born, than to the Word, which has begotten, conceived, and borne the church. We, through God’s grace, are not heretics, but schismatics, causing, indeed, separation and division, wherein we are not to blame, but our adversaries, who gave occasion thereto, because they remain not by God’s Word alone, which we have, hear, and follow".