Its researchers started by estimating of the number of Christians who died as martyrs between 2000 and 2010 - about one million by their reckoning - and divided that number by 10 to get an annual number, 100,000.
But how do they reach that figure of one million?
When you dig down, you see that the majority died in the civil war in the Democratic Republic of Congo. More than four million are estimated to have been killed in that war between 2000 and 2010, and CSGC counts 900,000 of them - or 20% - as martyrs. Over 10 years, that averages out at 90,000 per year.
So when you hear that 100,000 Christians are dying for their faith, you need to keep in mind that the vast majority - 90,000 - are people who were killed in DR Congo.
This means we can say right away that the internet rumours of Muslims being behind the killing of 100,000 Christian martyrs are nonsense. The DRC is a Christian country. In the civil war, Christians were killing Christians. The civil wars in the DRC were the consequences of a failed state, disintegrated military force so that militias had almost full power because of the weapons they had. They were indiscriminately killing and raping and plundering and it's very difficult to describe any of that killing as creating martyrdom. Surely it's not the case that all actively practising Christians who are killed in a civil war, are killed because of their faith? Vatican reporter and author of The Global War on Christians, John Allen, outlines an example of how someone caught up in the civil war in DR Congo could be martyred.
"A female catechist in Congo, who is having success persuading young people in her area not to sign up with the militias, and she is killed by one of those forces because they don't want to see the sources of recruits dry up. Now is that anti-Christian violence, or isn't it?" he asks.
In earlier estimates of martyrs, CSGC included killings that occurred in the Rwandan genocide. Again this is puzzling. It was not a conflict about religion - it was a case of Hutus killing Tutsis, and both sides were Christian.
"The genocide in Rwanda was based on the systematic killing of an ethnic group in an attempt to completely wipe them out and it had nothing to do with the beliefs or the worship or the people who were killed," says Ian Linden, author of Church and Revolution in Rwanda, and associate professor in the study of religion at the School of Oriental and African Studies in London. Ian Linden also makes the point that there were Hutus in Rwanda who wouldn't leave their Tutsi colleagues because of their Christian faith, and who were therefore killed and could be called martyrs.
The truth is two thirds of the 2.3 billion Christians in the world today live in dangerous regions. They are often poor. They often belong to ethnic, linguistic and cultural minorities. And they are often at risk.