"And he said, It is a light thing that thou shouldest be my servant to raise up the tribes of Jacob, and to restore the preserved of Israel: I will also give thee for a light to the Gentiles, that thou mayest be my salvation unto the end of the earth."
-- Isaiah 49:6
This verse is the base of the principal of or la'goyim -- "a light to the gentiles", or, as it is more often translated, "a light unto the nations". This is the idea that Judaism is the path to heaven, and that therefore one should not hide one's faith. Activities that demonstrate your faith are encouraged, or even required -- although this duty has historically been waived when this might put a person or family in danger. Examples of or la'goyim include congregations volunteering in the community, individuals inviting neighbors to Passover Seder, and perhaps most traditionally, placing the lit hanukiah where its lights can be seen from the street.