You may recall the name Ron Wienland. He made a prediction that the Rapture would take place on May 27,2012. People sold their homes and their cars. They took out their life savings and borrowed huge sums of money because they believed this supposed prophecy. In order to believe in any prophecy we need a certainty of past prophecies. If Ron had made a prophecy that was fulfilled then people could believe his next prophecy much easier. Ron failed in his prophecies and that gives Ron no credibility in any prophecy he will state in the future.
When we look at Isaiah 42 we see clearly that the Messiah is being foretold. The fulfillment of past prophecy brings proof that new prophecies from God would also be a reality. Israel was to be separate from the rest of the world, that was why they had no king before Saul, and that was why they had different rules for living. They were to be different, and to be a light to the Gentiles, that the Gentiles would believe. Israel failed in that mission, but God says in v6 that He would keep his covenant, or His promise to Israel, that they would be that light, and they would release the Gentiles from their prison of sin.
This verse tells us that God odes not leave us in the dark, we are told everything we need to know to go through life. It is not the case of something happening and God then telling us, "Oh, right, I was going to tell you that, but I forgot." All of the things that God had foretold of Israel had been fulfilled, and He now tells the Israelites through Isaiah that the Messiah will come, and you can be sure that it will happen because the previous things have been fulfilled.
The Messiah did come, He bled, and died, and suffered on the cruel cross of Calvary for you and for me. It was me who was to suffer for my sin, but it was Jesus who took that punishment for me. How grateful we can be that Jesus Christ paid that death penalty for us. The Messiah was here, and all of the prophecies concerning His first return have been fulfilled. Now we look forward to His Second Coming and because the prophecies of His first have been fulfilled, we can be sure that the Second Coming will happen.