The missionaries will say: Isaiah 49:6 shows that the Messiah is be a light onto the nations.
The Reply is: They do not say how Jesus fulfills this role, however, the very text itself does not support this as being the Messiah. It is simply not a Messianic prophecy. The full verse says "For He has said: "It is too little that you should be My servant in that I raise up the tribes of Jacob and restore the survivors of Israel: I will also make you a light of nations, that My salvation may reach the ends of the earth." Again, Israel is the servant. See above also Isaiah 42 and Isaiah 52-53 on Israel as the servant of G-d.