Lead means most followers and fastest growing.
Those are two different things.
As for the most followers:
It's more or less a tie between Christianity and Islam in terms of sheer numbers- Christianity with some 2 billion, and Islam with 1.3 billion.
But, in actual "true believers", Islam, I suppose, takes the cake. A lot of Christians are lapsed somewhat.
But, "fastest growing" is an entirely different matter.
Hypothetically, if you have a religion that has, say, 200 members, and increases to 2000 the next year, it'd be growing at a 1000% rate, which is phenomenally fast, but insignificant in terms of sheer numbers,. If a religion that has 1,000,000 members in one year, and grows to 1,001,800 members by the next year, it will be shown as growing slowly, even though it is gaining the same number as the smaller one.
So, going by various sources, I'd say in terms of a theistic path which is fastest growing, Wicca and neopaganism are expanding at a rapid pace, with at least 780,000 followers in the US and Canada alone, and another few million worldwide- according to a 2005 poll, which can be viewed on adherents.com. These numbers are greatly expanded from the numbers shown in previous polls in 2000 and others in the 1990s.
In terms of non-theistic religions/philosophies, Humanism and secularism are increasing rapidly, and count some 1.2 billion people who identify as such. This is about 20% of the world population, which is increased from earlier poll results.