Many people who have relationships with xtians claim they are being a "light unto the nations" yet they are not telling the xtians they have to give up xtianity. Last I checked the "light" that we are suppose to give to the nations is the Torah and the belief in one G-d. Being a "light unto the nations" is not a commandment, it is a fact. Someone once described it to me this way: Am Yisrael is like a lighthouse that shines a light to show the way, it is up to the lost to go towards the light. When we as a nation are living by Torah we are a light, by example. We were never meant to go out into churches to bring people out of idolatry. That would be analogous to jumping into a deep pit to pull someone out instead of throwing down a rope. In the end we get stuck in the pit with them.
A must read :
WHY CHRISTIANITY IS WRONG EVEN FOR GENTILES