Many years ago in the mid-90's as a regular pioneer and ms, I studied with a college student that was majoring in philosophy and a minor in the world's religions (or something like that). His thesis was on the history of the beliefs of jws. I foolishly thought that even if not sincere in his efforts, I could help him see the "truth". Only after leaving have I come to appreciate the impact this guy had on me and teaching me critical thinking skills. One occasion in particular stands out as an epiphany. I had the P.O's son, a regular pioneer also, on the study with me. This brother read a quote regarding the trinity, a major topic of our discussions, directly from the reasoning book as if it were the final authority. I remember himm saying, "see it says it right there, what else do you need?". I was so embarrassed by that, because by that time I realized that this student wouldn't just accept what WT said as fact, something all of us did accept. I was very uncomfortable and so embarrassed as I watched the student just smile and shake his head. I look back on that day now as the first crack in the facade. It changed my entire approach to my ministry and my talks. I never again relied on statements from WT as proof. It seriously affected my confidence in teaching and I never felt very comfortable dealing with topics like the trinity again.
Its funny how lost I felt as the student refused to study from a publication. We had to just use the bible. I'm so thankful now for the things he taught me, that frankly I hardly realized and certainly didn't appreciate at the time. I was 100% defeated by him.
So planting seeds of doubt are effective but may take time to mature. It took me 20 years.