Some features that that must be achieved for it to count:
The movement must have clearly defined doctrines or membership criteria. In other words, you can’t just 2 vaguely similar movements with 5,000 US followers and claim that’s equivalent to 10,000 followers. The movement must have clear criteria for membership that 10,000 people meet.
The number of people can be inferred, it does not have to be empirically demonstrated if it is otherwise clear
The movement does not need to be founded after the creation date of this market, it only needs to reach 10,000 US followers before the cutoff date.
The movement can’t be a part of any existing religion. This can include formal affiliation with existing churches but also anything that is non-denominational but too similiar to existing traditions to count. For example, I would reject any Christian movement that adheres to the nicene creed from consideration unless it also ties in multiple teachings that are clearly inconsistent with Christianity. Hypothetically, I would count Mormonism but not count Kenneth Copeland ministries if they had reached 10,000 followers within the market’s timespan.
In order to count as a religious movement, it must be centered around God or Gods.
1,000Update: https://en.wikipedia.org/wiki/Christian_fellowships_of_%22the_Remnants%22_movement could be a potential contender though it seems difficult to know if it had 10,000 followers before the market started
https://claude.ai/share/31a27431-b3f1-479d-b5e4-49e440c4a821 according to Claude Opus 4.7 the last time a new religion managed to do this anywhere on Earth was with Falun Gong and Eastern Lightning, both of which were in 1990s China.
In the US, I would guess the last time was Scientology, with the last before that being Mormonism or Jehovah’s Witnesses depending on how heretical you think something has to be for it to no longer be considered Christianity