"I have declared, and have saved, and I have shewed, when there was no strange god among you: therefore ye are my witnesses, saith the LORD, that I am God." Isaiah 43:12
I heard a story about this guy who was a Christian and had a job that he worked at for years before anyone even knew he was a Christian. What is wrong with that picture? How can you work in a place day after day, year after year and no one have any idea that you are a Christian? As we said a few days ago, everything we do is reflective of our walk with Christ- it is inseparable.
I am not suggesting that you get a bull horn and make a loud announcement that you are a Christian. What you are will be evident by the way you conduct yourself, by your lack of compromising your belief in certain situations, by your speech, by your integrity, by the way you treat others, by your work ethics, by showing compassion, and so on.
Some people's defense is that they are "letting their lights shine, and lights don't do anything but shine". There is nothing wrong with that- we are to let them shine, people should "see" Christ in us. But that is only half of our witness- they should also "hear" something from us. We are called "witnesses". There are more scriptures that tell us to be vocal with our faith than there are that tell us to demonstrate it only. We have to do both- we can't help but do both.
Do people around you, do your acquaintances, does your family, do those people you rub shoulders with day in and day out know that you are a Christian? The reason we should let them know (be more vocal- not obnoxious) is not about "us". It's about lifting Christ up before them so that they might come into a relationship with Him.
Have a great day. Others should be able to tell that you are a Christian if you are around them for any length of time, and if they can't... why?
For further reading:
Isaiah 43:10; 44:8; 52:7