As others have observed questions are possibly best, where the heavy lifting is left to them.
The "Socratic method".
Personally I would ask them, on separate occasions, to explain to me:
- (Paul on) Faith vs Works
- (Paul on) Law vs Grace
- (The Watchtower on) "legalism"
- (Paul on) how righteousness or a righteous standing is attained
- (Paul on) what is the gospel