Read this: Despite Martin Luther's harsh polemics against his Roman Catholic opponents over issues concerning Mary and the saints, theologians appear to agree that Luther adhered to the Marian decrees of the ecumenical councils and dogmas of the church. He held fast to the belief that Mary was a perpetual virgin and the Theotokos or Mother of God. Special attention is given to the assertion that Luther, some three-hundred years before the dogmatization of the Immaculate Conception by Pope Pius IX in 1854, was a firm adherent of that view. Others maintain that Luther in later years changed his position on the Immaculate Conception, which, at that time was undefined in the Church, maintaining however the sinlessness of Mary throughout her life. For Luther, early in his life, the Assumption of Mary was an understood fact, although he later stated that the Bible did not say anything about it and stopped celebrating its feast. Important to him was the belief that Mary and the saints do live on after death. "Throughout his career as a priest-professor-reformer, Luther preached, taught, and argued about the veneration of Mary with a verbosity that ranged from childlike piety to sophisticated polemics. His views are intimately linked to his Christocentric theology and its consequences for liturgy and piety." Luther, while revering Mary, came to criticize the "Papists" for blurring the line, between high admiration of the grace of God wherever it is seen in a human being, and religious service given to another creature. He considered the Roman Catholic practice of celebrating saints' days and making intercessory requests addressed especially to Mary and other departed saints to be idolatry. His final thoughts on Marian devotion and veneration are preserved in a sermon preached at Wittenberg only a month before his death:

Which pope was responsible for the dogmatization of the Immaculate Conception?
What is the answer? (If it cannot be answered, return "unanswerable")
Pope Pius IX