Christianity is the fastest growing religion in absolute growth, meaning the fastest growing religion in the annual increase of new adherents. The absolute growth number is calculated as the annual number of new Christian converts subtracted the number of ex-Christians + number of births subtracted number of deaths (new converts - former Christians + births - deaths). Christianity is the fastest growing religion at both parameters, both in absolute number of new converts and absolute number of new Christians due to population growth. In all, this sums up to an additional approximately 25.2 million new Christians annually. 22.7 million due to population growth and 2.5 million due to conversions.http://fastestgrowingreligion.com/fgr.html
The following is taken from "Religious Projections for the Next 200 Years by Dr. Todd M. Johnson, Jun 30, 1995". This link will take you to the article and the associated tables. http://www.wnrf.org/cms/next200.shtml
"In summary, Christianity increased from 33.2 percent in 1990 to 37.9 percent by 2200, while Islam also increased from 17.7 percent in 1990 to 22.6 percent by 2200."
This would seem to indicate that Christianity is still outselling Islam on the Religion Market, but the increase in both faiths is probably due to the population explosion rather than to any spiritual awakening.
By percentage, conversion, or in raw numbers? Many different religions currently or previously have claimed to be the fastest growing religion. The world's largest religions that are showing increases that outrun birth-rate include Christianity, Islam, and Hinduism. There is often little coverage of the "unaffiliated" category (which includes agnostics, atheists, deists, and theists), although some evidence suggests this group is growing rapidly (see "Nonreligious" below). "The" fastest growing religion depends on the definition, such as absolute number vs. percentage, conversions only or also births, etc., how broadly a religion is defined (e.g., Christianity as a whole, or a particular denomination), and the period and region in question. There is no definitive answer to the question, nor can there be, as there are many types of Islam, Christianity and Hinduism. I do not consider "not religious" to be a religion, but this cohort of people appears to be slowly growing in the developing countries. However, in areas where non-religious people live, there are still outbreaks of religious conversion of all kinds.