if he is thirsty, give him water to drink.
In doing this, you will heap burning coals on his head,
and the Lord will reward you. NIV Proverbs 25:21-22
Theological debates inherently render defensive postures. Walls come up. Ears shut down.
By engaging in a debate, in many ways you are asking your opposition to put on full display their most cherished and private beliefs. Furthermore, you're prodding them to defend those beliefs.
Not very many people desire the truth more than they desire being right, although many people are deceived that true and right are one and the same.
So you've got your opposition in a vulnerable position, and they will naturally coil up like an armadillo to protect themselves. Your goal should be to get them to relax their guard as much as possible, because that is your best chance of making an impact.
Sure, there are circumstances where haughty rebukes actually do work to motivate people, but they tend to be very limited to situations where there is an inherent structure of respect, like a parent and child, coach and athlete, teacher and student, etc. You probably don't have that kind of relationship with your opponent. Instead, you're probably just some dude, or dudette, on the internet to them. Without that foundational structure of respect, brash language will only give them all the more reason to completely dismiss anything you say.
So be like Jesus! OK, not the real Jesus that we find in the Gospels, but that romanticized version of Jesus who many Christians worship today. You know, the one who is meek and humble, yet firm. Be kind. Be considerate. Try to lovingly correct them. Imagine you were having this conversation in front of your grandmother, or even with your grandmother, if you have to. The bottom line is that you need to show respect, and be nice!
After the debate is over, they will think about you and what you said. If you've been a jerk, made personal attacks instead of sticking to the subject matter, called them or their beliefs by offensive names, chances are they aren't going to really reconsider what you've said. But if you are nice to them, and you show them (even undeserved) respect, you just may be heaping hot coals on their minds. Because, if nothing else, they will be forced to wonder why someone so nice and reasonable has so many "misunderstandings", or is in collusion with Satan, or is destined for Hell.
That juxtaposition of a nice person inheriting an eternally negative afterlife is not going to be a comfortable thing for them to ponder...