It's often claimed that 100,000 Christians are killed every year because of their religion. Earlier this year, the Vatican called it a credible number. But is it?

Gunmen on motorcycles kill three people when they open fire on a wedding outside a Coptic Christian church in Cairo. A twin-suicide bombing outside a church in Pakistan kills at least 75 people.

These two recent headline-grabbing attacks occurred within just a month of each other. Horrific, but by no means isolated incidents.

So how widespread is anti-Christian violence?

"Credible research has reached the shocking conclusion that every year an estimate of more than 100,000 Christians are killed because of some relation to their faith," Vatican spokesman Archbishop Silvano Maria Tomasi announced in a radio address to the United Nations Human Rights Council in May.

On the internet, the statistic has taken on a life of its own, popping up all over the place, sometimes with an additional detail - that these 100,000 lives are taken by Muslims.

More or Less: Behind the stats Listen to More or Less on BBC Radio 4 and the World Service, or download the free podcast Download the More or Less podcast More stories from More or Less

The number comes originally from the Center for the Study of Global Christianity (CSGC) at Gordon-Conwell Theological Seminary in the US state of Massachusetts, which publishes such a figure each year in its Status of Global Mission (see line 28).

Its researchers started by estimating 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.

A broad definition Christian martyrs are defined as "believers in Christ who have lost their lives prematurely, in situations of witness, as a result of human hostility"... The largest martyrdom situation today is in the Democratic Republic of Congo, where atrocious acts of violence began in the late 1990s and have continued to the present... Although not all their circumstances would be considered "situations of witness," we estimate that a substantial proportion of those who have died meet our definition of martyr. Todd Johnson: The case for higher numbers of Christian martyrs

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.

"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."

But 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.

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.

Religious harassment (2006-11) Number of countries where religious groups were harassed: Christians - 145

Muslims - 129

Jews - 90

Others - 75

Folk religionists - 47

Hindus - 32

Buddhists - 23

Any of the above - 185 Pew Research Center Religion & Public Life Project, June 2013

So let's agree that some people were killed because of their Christian faith in DR Congo. Is it plausible that 20% were?

Todd Johnson, director of CSGC, told the BBC this figure was drawn from the 1982 edition of the World Christian Encyclopedia, which estimated that on average 20% of African nations were actively practising Christians.

But surely it's not the case that all actively practising Christians who are killed in a civil war, are killed because of their faith?

Johnson says his centre has abandoned this statistic in its more recent work. The 100,000 figure still appears in its 2013 Status of Global Mission, though.

And this brings us to another problem - while violence continues in DR Congo, it's less extreme today than it was at its height.

"That's a weakness of this approach," Johnson concedes.

"Even in the DRC things are not as intense as they were 10 years ago. Every year now it probably should go down. So it's probably decreasing year by year right now, but the method is not exact enough to [make those adjustments], so I've just kept it at 100,000 the last couple of years but I'm likely going to have to lower it unless something comes to our attention."

If you were to take away the 90,000 deaths in DR Congo from the CSGC's figure of 100,000, that would leave 10,000 martyrs per year.

More from the Magazine The Democratic Republic of Congo is potentially one of the richest countries on earth, but colonialism, slavery and corruption have turned it into one of the poorest, writes historian Dan Snow DR Congo: Cursed by its natural wealth

And the number of Christians who are killed each year because they are Christians is more likely to be in that order of magnitude, according to Professor Thomas Schirrmacher from the International Society for Human Rights.

"One has to see that there is no scientific number at the moment. It has not been researched and all experts in this area are very hesitant to give a figure," he says.

"We are starting a research project with several universities worldwide on this topic and there we start with a guess of 7-8,000 Christians killed as martyrs each year."

But to some extent this number crunching is besides the point for author John Allen.

"I think it would be good to have reliable figures on this issue, but I don't think it ultimately matters in terms of the point of my book, which is to break through the narrative that tends to dominate discussion in the West - that Christians can't be persecuted because they belong to the world's most powerful church.

"The truth is two thirds of the 2.3 billion Christians in the world today live… in dangerous neighbourhoods. They are often poor. They often belong to ethnic, linguistic and cultural minorities. And they are often at risk.

"And ultimately I think making that point is more important than being precise about the death toll."

Follow @BBCNewsMagazine on Twitter and on Facebook