This question really gave me pause to think. Heb 11 & 12 kept coming to mind... esp Heb 12:1-2 "Wherefore seeing we also are compassed about with so great a cloud of witnesses, let us lay aside every weight, and the sin which doth so easily beset us, and let us run with patience the race that is set before us, Looking unto Jesus the author and finisher of our faith; who for the joy that was set before him endured the cross, despising the shame, and is set down at the right hand of the throne of God.
How can we do any less if we're going to be called by His name?
I liked a lot of the comments by the others about why we don't speak out. The US is such a hotbed of "political correctness" that sharing our faith is considered discriminating against other beliefs (huh? we can't have our own beliefs because it might step on someone else's toes? But they can have theirs and we can't say anything?) Fear of rejection, fear of not having all the answers - in a sense, even churches give us fear of witnessing because we're expected to take classes on evangelism (been to some myself!) before we're "qualified" to share out faith. Heck, the disciples never took any classes on how to talk about what Jesus had done for them! They just told others about what they saw, heard, and witnessed themselves!