Look Naaji. You can't just read one passage and interpret it. Did you even read on to see what Jesus did next after Matthew 15:24? The Gentile woman asked Jesus to save her daughter, but Jesus said He only came for Isreal and not the Gentiles. When He said this, He said it to test His desciples because they were Jews and Jews did not like Gentiles. After Jesus said, "I am not sent but unto the lost sheep of the house of Israel," the woman still showed faith in Jesus and Jesus healed her daughter showing the decsiples that He was here for the world and not just Jews.
Jesus said to her, "It is not meet to take the children's bread and cast it to the dogs" meaning He was the children's(Isreal's) bread and the Gentiles were the dogs. But again He did not mean this. Read on to see what the woman said. Jesus was teaching His desciples a lesson.
In Matt 10:5-6 Jesus told them not to go to the city of the Samaritans because they first had to handle Isreal. When Jesus resurrected, He then gave them permission to spread the gospel to the world(Remember Jesus said, "Go unto other nations and baptize in the name of the Father, the Son and the Holy Ghost"). If you read books like 1 Peter and 2 Peter, you would see that after Jesus ascended to heaven, His desciples went forth into Gentile nations. But first they had to handle their own people. If you were at a certain place and among a certain people, would you not tell them the good news before you tell other people?
Finally Naaji, why would Isreal be lost? I thought they were following God's LAWS. They couldn't be lost since the laws of God were their main focus and not Jesus dying for their sins. This shows they were lost and needed to be saved. They needed a Saviour. That's why Jesus came. If Jesus did not die on the cross, what did He accomplish? What lost sheep did He save if He did not die on the cross?