Why does God say Israel is not to assimilate with the Gentile nations?

Les teaches that Israel wants its economy to mesh with the European common market and to compete with the Gentile nations, but God says no because Israel is a separated people. They have not learned their lesson. As they try to assimilate and become just another member of the family of nations, they get deeper into the hole they are digging for themselves and are backed into a corner. Soon Israel will have no hope except the coming of their king, and he will come. But first they will accept the false king, the Antichrist, as Jesus indicated in John's Gospel: they rejected Him when He came in His name, but they will embrace another who comes in his own name. Jewish people describe a Messiah with military, economic, and charismatic superman qualities, which describes the Antichrist, though they do not see it. What is left for the nation is their fullness; the Old Testament covenant promises are still coming, withheld for almost 2,000 years, but God has not forgotten them.