This is an inevitable aspect of life: people will always disagree with you.  What’s very important is how we ought to respond to someone when we disagree.  Here are few points I’d like to share from experience:

1. Don’t get angry. We love to use ad hominem attacks but remember you’re discussing an argument or position, not the person.  Getting upset is a natural reaction.  When you let your upset disrupt the friendly atmosphere or affect your arguments, STOP.
2. Stick to the arguments (following 1). Be reasonable and calm.
3. Go to the person with him you disagree with first.  This is simply Matthew 18.  Don’t write open letters with defamatory comments and unnecessary attacks (a la Norman Geisler).
4. Do your best to really have a robust understanding of the other position.  You often heard that you should know the other position just as well as you know yours if you want to criticize it. Well, that’s not true and it’s completely infeasible.  It surely helps but here why that phrase is a problem.  To offer criticisms you just need to contrast it with what you believe to be true.  This is simply conversion, contrapositions, obversions, contraries, and contradictions put into play. If you can do that then you don’t need exhaustive knowledge.  If that phrase is true then everyone will be shutting their mouths all day long.
5. Let the other person speak and don’t interrupt.
6. Don’t respond if you’ve been emotionally compromised.  Respond when you’re thinking clearly and calm.
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