This question is a very good one, but it is too dangerous for Jehovah's Witnesses to answer. The Watch Tower tells people that Charles Taze Russell was the first true christian since the first century congregations. This implies that for some 1,800 years there were no true christians acceptable to Jehovah.
To say that there were christians acceptable to Jehovah between these two time periods is to imply that Jehovah found non-Witnesses acceptable. This would further imply that you could be acceptable to Jehovah without being one of Jehovah's Witnesses or being part of an earthly organization. So, this question presents a catch-22 scenario.
The Watch Tower Society usually sees such questions as a "trap" to be avoided because, what ever the answer is, it will expose their false teachings. So, rather than answer it, they will make a statement like: "Isn't more important to considered what we need to do now to gain salvation?" It's a smoke-and-mirrors approach to the bait-and-switch.