Freedom of speech is a queer thing really. At one hand you are exercising your right to speak and express your opinion but by doing so you are sometimes criticising someone else's opinions and that can be hurtful for some people. First of all, let's clear up one thing here: DO NOT HURT SOMEONE ELSE'S FEELINGS INTENTIONALLY PERIOD.

Now comes the question of genuine critique of others opinions, arguments or sometimes even believes. When you are making a point against someone's opinion no matter how absurd it is, be as humble as possible. Your humbleness opens a window of understanding between you and the other party. Now when it comes to believes, the matter becomes rather more sensitive. People don't like their believes being shattered so be very cautious when it comes to believes. But no matter what, if you have something to express just do it in the humblest way possible.

Hope it helps. Thanks!