Martin Luther, a Catholic monk who rebelled against the malpractices of the Pope and hegemony of the Vatican said ''I have not left the Catholic Church, the Catholic Church has left me'', believed in almost all practices and rituals of Catholicism, such as infant baptism, veneration of Virgin Mary, use of idols and crosses in prayer, transubstantiation during the Eucharist, etc. He even considered the prominent figures of the Protestant Reformation, such as Ulrich Zwingli, John Calvin, etc as deviants. He just challenged the infallibility of the Papacy and absolute authority of the Vatican in socio-religious affairs. Was Luther a 'Protestant' or a Traditionalist Catholic?
Luther started out as a traditional Catholic and became a very zealous one, but as the name ‘Protestant’ was not coined until 1529 (due to the protest at the Diet of Speyer) your question can only be answered in light of that progression of event.
Luther was born in 1483, brought up as a Catholic, then made a voluntary vow to God when aged 21, while a student at the University of Erfurt. This was occasioned by being struck to the ground due to a bolt of lightning. He cried out to the patron saint of miners, “Saint Anne! Help me! I will become a monk!” Two weeks later, he had entered a strict Augustinian monastery as a novice. Later, he was selected for the priesthood by his superior. Luther set himself to the pursuit of holiness. Sometimes he would fast three days on end, and he laid upon himself vigils and prayers in excess of those stipulated by the rule, writing:
“I was a good monk, and I kept the rule of my order so strictly that I may say that if ever a monk got to heaven by his monkery it was I. All my brothers in the monastery who knew me will bear me out. If I had kept on any longer, I should have killed myself with vigils, prayers, reading, and other work.” Here I Stand, Roland Bainton, p45 (Lion 1988)
After returning from a visit to Rome in 1510, he was transferred from Erfurt to Wittenberg. He was appointed a professor at the university in 1511. Under the guidance of his spiritual mentor, Dr. Staupitz, he advanced into the mystic system of Catholicism and became a Doctor of Theology. This is all part of the case for saying that Luther was a staunch Catholic of the most traditional type.
However, we all know that that changed after he posted 95 points for discussion on the eve of All Saints in 1517. The posting of his affirmations was traditional, however. That was the way scholars would be invited to debate issues. Luther made no attempt to disseminate his complaints among the people. Others surreptitiously translated the theses into German and gave them to the press. Luther sent a copy to Albert of Mainz, who then forwarded them to Pope Leo.
There is no need to detail the events that led to Luther being excommunicated by the Pope. On 10 October, 1520 the papal Bull reached Luther and 60 days later, he was officially put out of the Catholic Church with much burning of his books, and a curse. However, the title “Protestant” could not be applied either to him or those who agreed with his theological stance until the Diet of Speyer in 1529, after which they were first called “Protestants”. After all those events, to ask if Martin Luther was a Protestant is a bit like asking, “Is the Pope a Catholic?”