When God spoke to the prophet Ezekiel, He told him in plain, straightforward terms what would happen if Ezekiel didn't speak up about sin in his society. Here's what God said to Ezekiel:
"'Son of man, I have made you a watchman for the house of Israel; so hear the word I speak and give them warning from me. When I say to a wicked man, "You will surely die," and you do not warn him or speak out to dissuade him from his evil ways in order to save his life, that wicked man will die for his sin, and I will hold you accountable for his blood. But if you do warn the wicked man and he does not turn from his wickedness or from his evil ways, he will die for his sin; but you will have saved yourself.
"'Again, when a righteous man turns from his righteousness and does evil, and I put a stumbling block before him, he will die. Since you did not warn him, he will die for his sin. The righteous things he did will not be remembered, and I will hold you accountable for his blood. But if you do warn the righteous man not to sin and he does not sin, he will surely live because he took warning, and you will have saved yourself.'"
Ezekiel 3:17-21, NIV
Are you thinking this doesn't apply to you because you're not a prophet? All Christians are "prophets" in the sense that a prophet's job is to speak the truth of God--and we are all called to speak God's truth at every opportunity.