Given [the rules of thumb from the Sit on It Model], how can a Christian have a non-toxic discussion about Christ with a non-Christian?
The answer would be, I guess, "Very carefully."
I keep ignoring the other factors that go into how a person regards a particular thing -- what am I up to now? Doubt, humor, charity, and firmness. They all affect how zeal and anti-zeal will interact.
Meanwhile, the rule of thumb in the dynamic post suggests that a zealous Christian may well cause at least temporary anti-Christian zeal by discussing Christ online.
What the model suggests, though, is something like this:
To win someone to his side, a zealot should try to increase both how highly the person regards the particular thing and how deeply the person feels about the subject.
If a zealot (at point Z in the diagram) wants the person he's talking to (at point A) to join him in his opinion and his depth of feeling, he can't just make him feel more deeply, since that might produce anti-zeal (at e.g. point B).
A dispassionate discussion with an honest partner can raise the partner's opinion to the point (e.g., C) where making him feel more deeply will have a good effect. I'd guess it's more natural, though, to increase both opinion and depth of feeling at the same time (shown by the meandering line from A to Z).
Keep in mind, though, that the Sit on It Model was developed to help describe how things go bad (and at that, it does little more than point out the obvious). If you want things to go well, better to refer to the P-F-A Model.