1 billion or about 10% of the world population within the given time frame of 100 years is an ambitious goal, but not impossible if you somehow gain momentum.
@Uriel is right that the market currently looks quite saturated, but there are two points worth noting:
- There’s almost a billion people who don’t expressively subscribe to religion at all (mostly agnostics and atheists).
- Out of the billions of formally religious people, many are so only on (church) paper, because they are secular in almost every aspect of their lives, but they have been born to (nominally) religious parents and may have been baptized (or passed an equivalent initiation rite) out of tradition only.
How do you get someone to join a certain religion when they …
- hadn’t ever been part of one before or
- would need to abandon their current one?
Make it unique.
Make it easy to join and simple to follow.
Make it beneficial to join, more beneficial to practice.
Make it compatible with existing religions, i.e. non-exclusive.
Have charismatic leaders who lead by example and will never exploit the possibilities that come with the call.
Don’t rely on a single prophet.
Don’t leave essential questions unanswered because that would lead to segmentation into sects sooner or later.
Don’t discourage popular traditions associated with religion, e.g. gift exchange for christmas.
Probably most importantly, don’t require belief in god(s)! Otherwise you couldn’t get any agnostics and atheists.
Encourage but don’t require followers to change their habits. Ideally, people would realize, prior to joining, that they already believe everything in the creed and have always tried to live by the resulting code of conduct.
To target christians, followers of other abrahamitic religions and agnostics/atheists from countries with strong christian tradition, one could start with the assertion that it’s not important at all whether there has been an actual Jesus (or Mohammed) or what relation he had with a celestial god, because (since we can never prove it) it also doesn’t matter whether that (single) god exists or ever existed. It’s okay to believe in them, though.
All that counts (today) is the Holy Spirit, the community. The point of the New Testament seen fro this perspective was that the other two parts of the christian Trinity vanished from Earth (with Ascension), if they ever were on it, and humanity was left to take care of it (similar to Genesis). Pentecost (or something like it) would therefore become the holiest or only festivity.
That’s compatible with essential practical beliefs of christianity that secular christians would agree with, but disagrees with several theological axioms, i.e. it would be considered “heresy” if claimed to be catholic etc., because it’s a subset that leaves out some essentials.
It does not (yet) sufficiently cover Islam, but it should be possible to pass Mohammed as a prophet.
Morals and ethics would build strongly on individual responsibility for the social and natural environment. Freedom and liberty would be stressed, but bound by the categorical imperative.
The practical benefits would mostly come from the resulting social network, not due to some promise of salvation.