Of all the major religions, Islam is by far the fastest growing religion, however this is purely due to the rate of population growth in the Muslim world compared with the decrease of population growth in the West.
Samuel Huntington in his book "The Clash of Civilizations and the remaking of world order," states the following:
"The percentage of Christians in the world peaked at about 30 % in the 1980s, levelled off, is now declining, and will probably approximate to about 25% of the world's population by 2025. As a result of their extremely high rates of population growth, the proportion of Muslims in the world will continue to increase dramatically, amounting to 20 percent of the world's population about the turn of the century, surpassing the number of Christians some years later, and probably accounting for about 30 percent of the world's population by 2025."
There have been suggestions by Christians that Christianity is growing at a faster rate, however this is not backed up by any data and it seems to be more related to Christians migrating from one denomination to another. For instance Christians who have traditionally worshipped in more formal denominations such as the Roman Catholic or Anglican churches have migrated to more informal Evangelical churches. From the studies that have been conducted, the vast majority of new members amongst Evangelical churches have moved there from other Christian denominations.