I'll take a stab... Slippery slop is probably the best fit, but I think this is also a case of Affirming the Conseqent.
"The first step toward apostasy is a drop in field service activity."
More formerly we could phrase this as (since the Watchtower is trying to apply that to you, the reader as a warning):
If a person is becoming an apostate their first step is not participating in field service.
You are beinging to not participate in field serivce
Therefore you are becoming an Apostate.
Compare this with:
If it's raining then the streets are wet.
The streets are wet.
Therefore, it's raining.
This is fallacious because it could be wet because it just finished raining, or it snowed/snowing and the snow melts, or it flooded, or a watermain broke, etc.
More formally, these statements can be summerized as:
If P then Q.
Q.
Therefore, P.
What is logically sound would be:
Modus Ponens | Modus Tollens |
---|
If p then q. p. Therefore, q. |
If p then q. Not-q. Therefore, not-p. |
So back to the JW statement. These would be acceptable:
If a person is becoming an apostate their first step is not participating in field service.
You are becomming an appostate
Therefore you don't participate in field service
and
If a person is becoming an apostate their first step is not participating in field service.
You are still fully active in field service
Therefore you aren't becoming an apostate.
But something tells me that looking at that last example, while logically sound, it's based on a premise that is probably also another logic fallacy, A hasty generalization. It could be possible the "first step" is something else (independent thinking, a scandel, intellectual honesty, etc) or that an "apostate" could be still going in field service to put up a facade.