Isa 42:6, Isa 49:6, Jn 1:9, Eph 5:8, Mt 5:16
In the Old Testament, it was prophesied that God would send Christ to this earth to be a light for the people of Israel and the Gentiles. (Isa 42:6, Isa 49:6)
Isa 42:6 I the Lord have called thee in righteousness, and will hold thine hand, and will keep thee, and give thee for a covenant of the people, for a light of the Gentiles;
Isa 49:6 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.
Christ, the light, has come to this earth. That light is Jesus. (Jn 1:9)
Jn 1:9 That was the true Light, which lighteth every man that cometh into the world.
By believing in Jesus as the Christ, we too have become a light in Christ. Also, we have become those who preach this light, Christ. (Mt 5:14-16, Eph 5:8)
Mt 5:14 Ye are the light of the world. A city that is set on an hill cannot be hid. 15 Neither do men light a candle, and put it under a bushel, but on a candlestick; and it giveth light unto all that are in the house. 16 Let your light so shine before men, that they may see your good works, and glorify your Father which is in heaven.
Eph 5:8 For ye were sometimes darkness, but now are ye light in the Lord: walk as children of light: