A certain method of da'wa or forbidding wrong may in fact save a person's iman by the permission of Allah. At the same time, it's quite possible that it *harms* another person's iman. Or turned them away from Islam permanently.
What if saved 1 and turned away 2?
We're obligated to change at that point.
Saving and harming people is all a judgement based on people's reactions. If you can judge that a messge saved someone, then you can also judge that you harmed someone. Just look at their reactions.
This aspect of da'wa is trial and error, provided doesn't exit Shar'i bounds.