Globally, the 2018 Status of Global Christianity reports Christianity is growing at 1.3%, outstripping the global population growth rate of 1.2%. Christianity is the third fastest growing religion in terms of annual growth rate: Islam at 1.94% and Hinduism at 1.33% are both faster in terms of annual growth rate. However, the times are changing, largely driven by changing demographics (nearly all religions gain ten times or more new members through births than through conversion, and Christianity is no exception to this). Here are the changing annual growth rates for the periods of 2000-2018 and 2018-2025:
This is further clarified by looking at the raw number of people added per year for each of the major religions:
Right now, Hinduism is second fastest in terms of annual growth rate (AGR), but third in terms of the number of people added per year. The AGR growth rate is a bit of an anomaly. By 2025, Islam and Christianity will be first and second both in terms of % AGR and raw population gains.
Now and through 2050, in any given year, Christianity and Islam are responsible for more than half of the religious growth in the world; with Hinduism, nearly all of it. The world is getting progressively more religious, and mostly either Christian or Muslim (the vast majority of Hindus are in South Asia alone). Agnostics and atheists may be increasing in some parts of the world, but globally they are losing “market share.”
The top position, both in terms of numbers added and annual growth rate, belongs to Islam–but by the thinnest of margins. Those margins are likely to be maintained through 2050, however.