Americans have been a loud bunch from the get-go, but today we live in an era that's perhaps louder than any before. Whatever your strongly held beliefs, it's more likely than ever that they're not secret. This can mean different things for different people in different careers and professions. Some employers are fully on board; others, really not so much.
Unless you've been a master at covering your tracks from the day you first logged onto something, your online presence can easily be traced to your professional sphere. In that case, it pays to know what your employer's policies on social media and protest are. There may be restrictions on appearing at a protest during work hours or wearing identifiable company logos. Your employer almost certainly has a social media policy. One study among recruiters found that more than half lowered their estimation of job candidates because of online "political rants."
In any of these cases, knowledge is power — and forewarned is forearmed. The most important thing to know is whether you're an at-will employee. In those instances, your employer can let you go for just about any reason. (Remember, your right to free speech is about the government stopping you, not private enterprises.) You should also check in with your management or human resources department about what a company's preferences on political speech are, if you haven't found it in an employee handbook.
Finally, it's becoming more common for activists to contact an employer about a worker's protest actions or online speech. Figure out how much your boss is willing to go to bat for you. If the answer is "not at all," that's useful information, even if it's disappointing. At that point, consider what's most vital to living your beliefs, and whether you might find other ways to make your voice heard.