To your question ziddina ... my experience has been that you can make a point that should be devastating to their doctrine, their eyes will glaze over and their heads will nod in acknowledgement, however the next words out of their mouths will betray the fact that nothing you said has had any impact.
Perhaps a waste of time. Until someone is open to change, it is difficult to have much influence on their thought processes.
Having said that, I'll keep trying as long as they keep coming around.