It depends entirely on how the question is read.
If the question is asking what is the largest religion that is
still growing, the answer would be Christianity since there are
more and more Christians born and converted every day than convert
out or die.
If the question is asking what religion is growing by the
largest numbers of new followers (both births and converts), then
the answer would be Islam, since the population growth in
Muslim-majority countries has made Islam the religion with the most
new adherents daily.
If the question is asking what religion has the greatest
percentage of growth, then this would be a contest between a number
of very small religions which have high percentage increases, but
low current population counts, such as Baha'i or Jehovah's
Witnesses. Although these cannot match the numerical growth of
Christianity or Islam, the thousands of converts and newborns have
a much higher relative to the <20 million adherents to the
religion.
