This is from a book by David A. Plaisted called, “Estimates of the Number Killed by the Papacy in the Middle Ages and later” You can download the entire document here.
Some of the text from that document:
Here are some of the places where figures about religious persecutions are given. Dowling in his History of Romanism says
“From the birth of Popery in 606 to the present time, it is estimated by careful and credible historians, that more than fifty millions of the human family, have been slaughtered for the crime of heresy by popish persecutors, an average of more than forty thousand religious murders for every year of the existence of popery.”
— “History of Romanism,” pp. 541, 542. New York: 1871.
Commenting on this quote, a fundamental Baptist web site says the following:
For example, it has been estimated by careful and reputed historians of the Catholic Inquisition that 50 million people were slaughtered for the crime of “heresy” by Roman persecutors between the A.D. 606 and the middle of the 19th century.
This is the number cited by John Dowling, who published the classic “History of Romanism” in 1847 (book VIII, chapter 1, footnote 1). Only seven years after its first printing, it could be said of Dowling’s book, “it has already obtained a circulation much more extensive than any other large volume ever published in America, upon the subject of which it treats; or perhaps in England, with the exception of Fox’s Book of Martyrs.” Clark’s Martyrology counts the number of Waldensian martyrs during the first half of the 13th century in France alone at two million. From A.D. 1160-1560 the Waldensians which dwelt in the Italian Alps were visited with 36 different fierce persecutions that spared neither age nor sex (Thomas Armitage, A History of the Baptists, “Post-Apostolic Times – The Waldensians,” 1890). They were almost completely destroyed as a people and most of their literary record was erased from the face of the earth. From the year 1540 to 1570 “it is proved by national authentic testimony, that nearly one million of Protestants were publicly put to death in various countries in Europe, besides all those who were privately destroyed, and of whom no human record exists” (J.P. Callender, Illustrations of Popery, 1838, p. 400). Catholic historian Vergerius admits gleefully that during the Pontificate of Pope Paul IV (1555-1559) “the Inquisition alone, by tortures, starvation, or the fire, murdered more than 150,000 Protestants.” These are only small samples of the brutality which was poured out upon “dissident” Christians by the Roman Catholic Church during the Inquisition.
Concerning the figure of two million killed, Bourne writes
Bertrand, the Papal Legate, wrote a letter to Pope Honorius, desiring to be recalled from the croisade against the primitive witnesses and contenders for the faith. In that authentic document, he stated, that within fifteen years, 300,000 of those crossed soldiers had become victims to their own fanatical and blind fury. Their unrelenting and insatiable thirst for Christian and human blood spared none within the reach of their impetuous despotism and unrestricted usurpations. On the river Garonne, a conflict occurred between the croisaders, with their ecclesiastical leaders, the Prelates of Thoulouse and Comminges; who solemnly promised to all their vassals the full pardon of sin, and the possession of heaven immediately, if they were slain in the battle. The Spanish monarch and his confederates acknowledged that they must have lost 400,000 men, in that tremendous conflict, and immediately after it-but the Papists boasted, that including the women and children, they had massacred more than two millions of the human family, in that solitary croisade against the southwest part of France.
— Bourne, George, The American Textbook of Popery, Griffith & Simon, Philadelphia, 1846, pp. 402-403.