I've also had the privilege of talking to different sets of Jehovah Witnesses. The first of those weekly studies went a year and a half, and the second went six months. Just like Mormons, I appreciate Jehovah Witnesses desire to further what they believe is true in a peaceful manner.
I didn't want to start with looking at historical context of the Witness movement, or the credibility/authenticity/integrity of its founder(s), or discussions regarding the vastly different Bible translation, or the foundation of the Watchtower/Awake magazines, etc. I wanted to start with Bible topics as the common starting point.
The discussions lead me to dig into several Biblical topics that I hadn't researched before. One topic was the Christian Godhead (Trinity concept); if you like, you can review the content I compiled during that research.
There were multiple topics where we held contrary viewpoints. However, the reason I cannot accept the Jehovah Witness testimony is because the following three points are historically accurate, reliable, and true without applying theology (i.e. from a non-christian standpoint):
- Jesus Christ claimed to be God
- Jesus was killed for that claim
- The early church held the same viewpoint
Why is this a showstopper? Because you can't accept some truths around Jesus and reject others; Jesus never left that as an option. Some writers explain this better than others (e.g. C.S. Lewis), which I won't go into here. The main point is that historical truths are not subjective.
It's very odd that Atheist historians (who widely accept only 6-to-7 Pauline epistles as historically accurate, reliable, and true) will say those points are valid... while Jehovah Witnesses (who claim to believe the rest of the Bible as true) reject it because it conflicts with their NWT translation.