Islam is set to surpass Christianity as the world's largest religion within this century, according to a report about the future of religious groups. The number of children within each family may be a key factor.

The Pew Research Center put together a religious population growth report in 2015 and updated their analysis last month after taking recent surveys into consideration.

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"If current demographic trends continue, the number of Muslims is expected to exceed the number of Christians by the end of this century," concludes Michael Lipka, a senior editor focusing on religion at Pew.

Why is Islam growing more rapidly?

"The main reasons for Islam's growth ultimately involve simple demographics," wrote Lipka and his colleague Conrad Hackett -- a demographer also focusing on religious issues. "To begin with, Muslims have more children than members of the seven other major religious groups analyzed in the study."

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Muslim families have an average of 3.1 children per woman, while Christian families have an average of 2.7 children, Pew found in 2015.

Christianity and Islam both trace their history back to the biblical figure Abraham. Abraham, called Abram at the time, could not bear children with his wife. God then promised Abram a son and told Abram, "Look up at the sky and count the stars -- if indeed you can count them. So shall your offspring be."

Today, Christians make up roughly 2.2 billion people.

"There were 1.6 billion Muslims in the world as of 2010 -- roughly 23 percent of the global population," the Pew Research Center estimates. Islam is currently the world's second largest religion; Christianity is the largest.

From 2010-2050, the Islamic religion's population size is projected to grow by 73 percent; the Christian population is projected to grow 35 percent in the same time frame. This would make Islam the world's fastest growing religion. By 2050, Muslims are expected to be nearly even in number with the Christian population.

The third largest religious group, according to Pew's research, is an "unaffiliated" segment that makes up about 1.1 billion people. The Hindu religion comes in next with about 1 billion people adhering to the practice.

In 2015, Muslims made up about 1 percent -- or 3.3 million people -- of the U.S. population, according to a best estimate by Pew.

"Living in a religiously pluralistic society, Muslim-Americans are more likely than Muslims in many other nations to have many non-Muslim friends," the study found.

As political and religious tensions rise worldwide, many countries, including the United States, have tried to combat threats related to radical Islamic terrorism.

"In many cases, people in countries with large Muslim populations are as concerned as Western nations about the threat of Islamic extremism, and have become increasingly concerned in recent years," Lipka wrote. He added, "More generally, Muslims mostly say that suicide bombings and other forms of violence against civilians in the name of Islam are rarely or never justified."