12 principles that are worth considering and praying over before you engage in online debate:
1) Do not respond hastily. Think and pray before you speak.
2) Be brief: You want to be clear—and to articulate your point without being pompous. Be direct. Stay on topic. Don’t lose yourself, or your readers, in overly wordy sentences or paragraphs.
3) Take the time to let people know you have understood their comments and concerns. Show respect to them by repeating their argument back in your own words and ask if you have understood them correctly.
4) Respect the opinions of others. When you need to disagree, do so respectfully and acknowledge valid points in other's arguments. While others are entitled to hold any perspective on an issue, be sure to point out their own inconsistencies, but with gentleness and respect (1 Peter 3:15).
5) Do not write anything that sounds angry or sarcastic. (James 1:19)
6) Never speak as if you are glad at the misfortune of another. Show care, concern or empathy.
7) When you reprove another, remember that you are not without sin, and are not beyond the capacity to sin. Always keep in mind that you are not saved because you are better or more deserving than others, but are saved by God's mercy alone. Any good character we may now have is only because of God's mercy. Don't forget that in your discussions.
8) Do not use reproachful language against others: curses, reviling or argumentum ad hominem: attacking the character, motive, or other attribute of the person making the argument, or persons associated with the argument, rather than attacking the substance of the argument itself.
9) Do not relate news that you are unsure of the truth of. Do not forward stories that are conjecture and speculative or "news" before the actual facts have come out. This often means you should consider withholding comment for a day or two after "news" has appeared on social media.
10) Avoid making orphaned assertions. i.e.... "Atheism is false." If you make these kind of assertions explain why you think it is so.
11) Use proper grammar and writing style: Do so as if you were writing an important paper. Correct spelling, grammatical construction and sentence structure are expected elsewhere. Online discussions are no different. This is a principle I am still working on.
12) Be patient with others. God works in his own time.
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Related Resources
Principals for Debate Resources @ Monergism
Before You Post...by Lane Keister
6 Principles for How to Argue When You Disagree by Tim Keller
Seven Thoughts about Engaging Well in Disputes by Darrell Bock