In Deuteronomy Moses is explaining to Israel how to operate in the world. They had all grown up in the wilderness and had no practical experience with the temptations and pressures of the 'civilized' world. He is very clear that they will face strong pressure to adapt and fit in. He gives them tools to defeat that pressure. God's purpose for Israel is for her to change the world into the Kingdom of God, not to become just like everyone else. So too with Christians.