(CNN) - Christians are by far the largest religious group on the planet, and the religion has gone truly global over the past century, according to a new report out Monday, which finds some of the world's biggest Christian communities in surprising places. Europe was the clear center of world Christianity one hundred years ago, but today the Americas are home to more than a third of all Christians. In fact, the United States has the world's largest Christian population, of more than 247 million, followed by Brazil and Mexico. China also appears on the list of top 10 largest Christian populations - with an estimated 67 million Christians, it has more followers of the faith than any western European country. There are nearly 2.2 billion Christians around the world, making up about one-third of the world's population - the same percentage as a century ago, according to the Pew Forum on Religion & Public Life. Islam is the world's second largest religion, with about 1.6 billion followers worldwide, the Washington-based organization calculates. That's just under one-quarter of the estimated 2010 world population of 6.9 billion. Sub-Saharan Africa has seen the biggest explosion in its Christian population in the past century, going from about 9 million Christians in 1910 to about 516 million today - nearly a quarter of all the world's Christians. Three of the world's ten largest Christian populations are in Africa: Nigeria, the Democratic Republic of Congo and Ethiopia. The study, "Global Christianity," is based on demographic and opinion data from 232 countries and territories. It's part of the Pew-Templeton Global Religious Futures Project, which has also included reports on the current and projected number of Muslims in the world. It does not measure practice or belief, merely counting as Christian anyone who says they are. The report calculates that half the world's Christians are Catholic, 37% are Protestants, and 12% are Orthodox. The remaining 1 percent belong to other traditions such as Mormonism.