The tenth parallel - the line of latitude seven hundred miles north of the equator - is a geographical and ideological front line where Christianity and Islam collide.

Across much of inland Africa and Asia, from Nigeria, Sudan and Somalia to Indonesia, Malaysia and the Philippines, live more than half of the world's 1.3 billion Muslims, and sixty percent of the world's 2 billion Christians. The space between the equator and the tenth parallel marks the end of Africa's arid north and the beginning of sub-Saharan jungle; in Southeast Asia, the encounter between the two religions is also driven by wind and weather, as the trade winds carried merchants of both faiths across the sea, and the clash of hot and cold air creates the hurricanes that travel across the earth to hit Latin and North American soil. On both sides of the line, the religions and their people are experiencing reawakenings of faith - and in their buzzing megacities and swarming jungle, the encounters between the two faiths is shaping the future.

Eliza Griswold, award-winning investigative journalist and poet, has spent the past seven years travelling the space between the equator and the tenth parallel, exploring the meanings and ramifications of this reawakening of faith, in a place where these changes may alter the future of what's called the Global South - and, in turn, the West. In each country along the faultline, she asks if it is possible to determine where faith ended and secular violence began, or what role religion actually plays in struggles over resources and political power. The story of this encounter between religions unfolds over nearly two thousand years and more than 600,000 square miles.

An urgent examination of the relationship between faith and worldly power, The Tenth Parallel is an essential work about the conflicts over religion, nationhood and natural resources that will remake the world in the years to come.