Is a claim of 50,000,000 Baptist martyrs accurate? In the book The Trail of Blood by James M Carroll, the author makes the claim that 50,000,000 early Baptists were martyred under persecution by the Catholic Church.  The author makes the claim multiple times without any sourcing for that number.  My question is essentially two parts.  Is the claim of 50,000,000 Baptist martyrs accurate, and if so, can someone point me to a source that validates the claim?
No, there is no reliable historical evidence to support such a claim.

Before going into the question significantly, I should point out that the number of Baptists killed by the Catholic Church depends on how one defines "Baptist" - apparently, there are some who consider any early sect of Christianity which did not practice infant baptism to be a "proto-Baptist"; a predecessor of the Baptists. Rather than decide "who really is a Baptist", I'm just going to count all Christians who were killed, or died, by order or complicity of the Catholic Church: to my shame I must admit there were many.

The first appearance of the number "fifty million" that I can find is in Charles Buck, A Theological Dictionary, Containing Definitions of All Religious Terms, 1836, in the article "Persecution":


  It has been computed that fifty millions of Protestants have, at different times, been the victims of the persecutions of the Papists, and put to death for their religious opinions.


Unfortunately, Buck does not state where he gets this number. In his discussion of "Persecution of Christians by those of the same name", in which he focuses exclusively on post-Reformation persecutions of Protestants by Catholics, he mentions the Inquisition, the Dutch Revolt (not by that name), the French Wars of Religion (likewise not by that name), the English Reformation, the 1641 Irish Rebellion, and unspecified slaughters by Catholics in Scotland and Spain (although it specifies that the Spanish killings it mentions are of Jews, Muslims, and "barbarians", not of Christians).

According to this source, which cites the "fifty million" number as well (and which refers to the statement by Buck), between fifty and sixty-eight million people (not necessarily all Protestants) were killed by "Inquisition", 1518 and later. This apparently refers to the Roman Inquisition, although it could refer to the Spanish and perhaps the Portuguese Inquisitions as well. This academic source states that relatively few of those tried by the Inquisitions were in fact executed; Wikipedia estimates that the total number of people executed by the inquisition was probably between three and five thousand. There are much higher estimates, but none higher than 350,000; the median of several reliable estimates is 32,000.

The Dutch Revolt appears to have killed between 50,000 and 100,000 people.

The French Wars of Religion probably killed about three million people (Wikipedia estimates between two and four million).

In the English Reformation, based on the Wikipedia list, fewer than 500 people were killed.

In the Irish Rebellion of 1641, about 112,000 Protestants were killed.

Wikipedia, citing an article in The Oxford Book of Scottish History, estimates that "over 1,500 people" were executed for witchcraft, of between 4,000 and 6,000 tried.

These numbers total well under 5 million. While this amount is lamentable, it is nowhere near the fifty million cited.

Perhaps one could add the figures referred to in this source from FaithAssemblyOnline.org, which includes the Albigensian Crusade and the Thirty Years' War. It appears that scholarly estimates of the deaths from the Albigensian Crusade amount to between 100,000 and 1,000,000, roughly in agreement with the Faith Assembly source. Wikipedia's article on the Thirty Years' War, however (citing Europe, A History by Norman Davis), estimates that it caused 8 million deaths on all sides.

Finally, since I am unable to find a good source for the number of deaths caused by the crusade against the Waldensians, let's assume that the number given in the Faith Assembly source (900,000) is correct. These numbers total 14.5 million, nowhere near the 50 million.

Note several things:


This is a total of all self-described Christians whose deaths might be ascribed to the Catholic Church, whether they're considered precursors of Baptists or not. 
In conflicts (the Dutch Revolt, the French War of Religion, the Thirty Years' War) for which separate numbers of Catholic and Protestant deaths were not given, I assumed that all people killed were Protestants. This is almost certainly quite far from the truth.
This is the highest possible estimate I could get from the numbers; the low estimate (taking the lowest estimate of non-conflict casualties, and assuming that casualties from conflicts were about half Protestant and half Catholic) is about 4 million.
The conflicts were in part caused by religious divisions, but they were political events as much as if not more than religious events; calling them "deaths caused by the Catholic Church" grossly misrepresents the causes and structure of the conflict, in my opinion. Nevertheless, I have included them to attempt to inflate the numbers as much as possible.


Based on my review of both the sources which provide the "50 million" number, and the historical evidence regarding the events to which the sources allude, I conclude that there is no historical reason whatsoever for believing that this number is anywhere close to accurate.