Using the hashtag #ExMuslimBecause, thousands of former Muslims are taking to social media to reveal why they left Islam - and some of the reasons are heartbreaking.

Islam is one of the largest religions in the world, with over 1 billion followers. According to the Pew Research Center, Muslims will grow more than twice as fast as the overall world population between 2010 and 2050 and, in the second half of this century, will likely surpass Christians as the world's largest religious group.

However, there are at least thirteen Muslim majority nations on earth where apostasy and atheism are punishable by death, according to a report from the Huffington Post. In addition, in Saudi Arabia, the birthplace of Islam and Mohammed, atheists are considered terrorists.

Those who are able to escape face physical torture, imprisonment and exclusion from the job market. In the West, ex-Muslims are often disowned from their families or even murdered by their own families in "honor killings."

Thus, last week, the Council of Ex-Muslims of Britain started the #ExMuslimBecause to encourage dissidents from across the Muslim world to come out and reveal why they left Islam.

"Those of us who have come forward with our names and photographs represent countless others who are unable or unwilling to do so because of the threats faced by those considered 'apostates' - punishable by death in countries under Islamic law," reads the group's manifesto.

"By doing so, we are breaking the taboo that comes with renouncing Islam but also taking a stand for reason, universal rights and values, and secularism."

Within just a few hours, the hashtag began trending across social media, with hundred of people sharing their personal stories of why they left Islam.

"#ExMuslimBecause there is no good reason to believe Muhammad. #ChristianBecause there is excellent reason to believe Jesus," tweeted New York Times bestselling author Nabeel Qureshi.

"#ExMuslimBecause I studied the life of Muhammad and the text of the Quran. History testifies against Islam and its truth claims," he tweeted a short time later.

"#ExMuslimBecause my own mother told me I should be killed because I didn't believe the same things she did," wrote another woman named Yas.

Wrote Eiynah, "#ExMuslimBecause I saw firsthand what growing up under Shariah Law is like in Saudi Arabia."

Wrote Imran Said, "#ExMuslimBecause there's a scary amount of people out there who think punishing people for apostasy and blasphem[ing] is perfectly acceptable."

While the campaign has condemned by some as "hateful", "Islamophobic" and in "bad timing" due to the Paris attacks, others, such as atheist and evolutionary biologist Richard Dawkins, have lent their support to the cause.

"Tell us why you gave up being Muslim. Go to #ExMuslimBecause. It's an encouraging, uplifting hashtag to read. Maybe there's hope", he tweeted.

Speaking to the BBC, Maryam Namazie, founder of the Ex-Muslim Forum, explained that she never expected the hashtag to begin trending, but said it was proof that people were eager to express themselves.

"Whilst we mourn our dead in Paris, we must not forget the countless others killed by ISIS and Islamists, including this very month in Lebanon, Nigeria, Mali, Iraq, Egypt, Bangladesh, Pakistan, Afghanistan... as well as those executed perfectly legally via Sharia laws in Iran, Saudi Arabia," she wrote in a later blog post. "The refugee crisis is in large part due to this unbridled brutality. In fact, if there ever was a "right" time to challenge Islam and Islamism, it is now".