What accounts for Islam being the fastest-growing religion in the world? (1) We will attempt to answer this by citing several statistics. Islam is growing at a pace that is expected to surpass Christianity by 2070 and become equal to the number of Christians in 2050 (2).
Pew Research Center reports that the main reason for the growth of Islam is due to Muslims having more children than members of other religions,
“Muslim women have an average of 2.9 children, significantly above the next-highest group (Christians at 2.6) and the average of all non-Muslims (2.2). In all major regions where there is a sizable Muslim population, Muslim fertility exceeds non-Muslim fertility… A larger share of Muslims will soon be at the point in their lives when people begin having children. This, combined with high fertility rates, will accelerate Muslim population growth.”
“People of course do change religious faiths, and we’ve measured religious switching, but for Muslims the story as far as we can tell is really about natural increase: a lot more Muslims being born than are dying in any given year” (3).
Most people who grow up in a faith retain that religious identity as adults, which, given that many more children are being raised as Muslims than we find in other religions, will favour Islam. It is expected that the greatest growth of Muslims will occur in Africa and the Middle East. The exception to this will be in Latin America and the Caribbean, where relatively few Muslims live and where Christians will remain the largest religious group (89%) in 2050. Moreover, in India, where Hinduism is dominant, Islam is growing at a faster rate than the country’s majority Hindu population. There Islam is expected to rise from 14.9% in 2015 to 19.4% by 2060.
In the United States, where there are around three and a half million Muslims, the Muslim population is expected to double by 2050 to around 8.1 million (4), and Islam will become the second-largest religion there by 2040. There is, however, little in the way of conversions to Islam. About one-in-five American Muslim adults were raised in a different faith or none at all, but this is counterbalanced by the number of people leaving the faith: “About as many Americans become Muslim as leave Islam.”
Beyond having more children, there are additional factors that could be contributing to Islam’s growth. We can suggest the following:
[1] There is coercion to stay in the faith. In many Muslim majority countries, there can be severe consequences for changing faiths. This could include stigmatization, violence, and legal consequences. Many will thus decide to remain Muslim to avoid these negative consequences.
[2] Islam is growing because many immigrants are moving to Western countries,
“In Europe, for instance, the Muslim share of the population is expected to increase from 5.9% in 2010 to 10.2% in 2050 when migration is taken into account along with other demographic factors that are driving population change, such as fertility rates and age.”
Islam is not the only religion expected to grow in the West because of immigration. Hinduism is expected to grow in the United States and Europe, and Buddhism is expected to grow in Europe. Immigration nonetheless brings Islam to new territories, enables Muslims to establish their faith in new locations, and share their beliefs with others.
[3] Islam’s growth is also a result of it being a missionary religion. It is due to its missionary nature that it spread so rapidly throughout the Middle East, across North Africa, and into Southern Europe. It also spread into South Asia, Southeast Asia, and in more recent times has penetrated Europe and the United States.
Missionary religions often include an apologetics dimension that tries to convince non-members of the reasonability of the religion. Muslim missionaries engage in various forms of apologetics including lectures and debates across the globe (easily accessible online and through DVD), translating their texts into local vernaculars for wider reach, and establishing missionary and apologetic organizations. Influential in Africa has been Ahmed Deedat’s Islamic Propagation Centre International located in Durban, South Africa (5). There are various other missionary and apologetic organizations in Europe and North America.
[4] Islam can offer an attractive religious alternative to many in the West. It might attract people because of its certainty regarding its outlook and guide to living life. Many people desire comfort in an uncertain and unpredictable world. Islam offers this on matters such as God, meaning, purpose, and life after death. It is also directive in that it instructs members on what they are and are not to do. We can expect this to be attractive to some Westerners who no longer or do not find the growing postmodern, relativistic mindset appealing. In postmodern, relativistic religions members can create their own realities, but many people do not think this is appropriate. Islam offers a religion that is grounded in a set scripture, a historical founder, and theology rather than in subjectivities and preferences.
References
- Pew Research Center. 2017. Why Muslims are the world’s fastest-growing religious group. Available.
- Pew Research Center. 2015. The Future of World Religions: Population Growth Projections, 2010-2050. Available.
- wbur. 2017. Why Islam Is The World’s Fastest-Growing Religion. Available.
- Pew Research Center. 2016. A new estimate of the U.S. Muslim population. Available.
- Haron, Muhammed. 2014. “Ahmad Deedat: The making of a transnational religious figure.” Journal for the Study of Religion 27(2).
Bishop's Encyclopedia of Religion, Society and Philosophy 
After Christianity has lost its values, only Islam offers some form of spirituality. Therefore, in order to survive, Christianity must merge with Islam.
Interesting perspective, however, I am not quite sure how that would work, especially given the claims made by Islam and Christianity. I also don’t think Muslims and Christians will want to do that either.