As of my knowledge cutoff in September 2021, the largest religion in the world is Christianity. It has an estimated 2.3 billion adherents, making up about 31% of the global population. Christianity is a monotheistic religion centered around the life and teachings of Jesus Christ. It encompasses various denominations, such as Catholicism, Protestantism, and Eastern Orthodoxy, among others. It is important to note that religious demographics can change over time, so it's always good to consult up-to-date sources for the most current information.

As of my knowledge cutoff in September 2021, Islam is often cited as one of the fastest-growing religions in the world. It has been projected that the Muslim population will continue to increase in the coming decades. Factors contributing to its growth include high birth rates among Muslim populations, as well as conversions to Islam. However, it is important to note that religious demographics can change over time, and there are also other religions experiencing significant growth in different regions of the world. For the most up-to-date and accurate information on the fastest-growing religion, it is advisable to consult recent studies and surveys.