So then, while we have opportunity, let us do good to all people, and especially to those who are of the household of the faith. (Galatians 6:10, NASB)
What exactly did Christ meant when He said "We should be like the shining light"? The early Christians were persecuted and killed for their faith, but that did not stop them from doing good to others. They didn't just care for the fellow believers... they cared for the nonbelievers as well. Here is an excerpt from an article.
Not a few times a plague would sweep through a city, and all the pagans left town immediately, leaving their loved ones to die. That included the physicians. But it was the Christians who stayed behind and tended to their needs, sometimes even dying in the process. (The Missio Dei by Frank Viola)
And here is a quote from an historical source to clear your doubts. In 362 the last pagan emperor, Julian “the Apostate,” wrote to a pagan priest, “For it is disgraceful that, when no Jew ever has to beg, and the impious Galileans [the Christians] support not only their own poor but ours as well, all men see that our people lack aid from us.”
What motivated these Christians to support and care for their pagan neighbors? Perhaps they took the words of Jesus Christ to heart.
So in everything, do to others what you would have them do to you... (Matthew 7:12, NIV)
