Who claims to know God and yet still thinks he can lead a life of his own, deceives himself and the truth is not in him, said the apostle John (1 John 1:6-8).
Israel was God’s people, it still is and it will remain so, as long as the sun and the moon will shine (Jeremiah 31:35,36).
But being one of the chosen people is no guarantee for living according to God’s will! God told Isaiah that the people of Israel did not know anymore what was right and what was wrong, although they knew the law by heart. Therefore He warned Isaiah not to identify with the people in any way (:12).
In the New Testament we are told something like that in several places. ‘You, however, did not come to know Christ that way (…) you were taught (…) to put off your old self’ (Ephesians 4:20,22) and, ‘Therefore, 'Come out from them and be separate, says the Lord. Touch no unclean thing'’ (2 Corinthians 6:17). When we really meet God in our lives, He will tell us roughly the same things as He did Isaiah, and He will warn us not to follow the ways of fake pious people like these.
But He - fortunately - does not only say what is forbidden, He also shows us the right way: ‘The Lord Almighty is the One you are to regard as holy, He is the One you are to fear.’ Then we will be safe under His wings, while others, who do not hallow God in their lives, will stumble, fall and be broken. The latter is a terrible judgment. But let us keep in mind that these words of God do not apply to the heathens, but to those who have heard the word of God, perhaps Sunday upon Sunday, but Monday upon Monday they have ignored it, as if God had not spoken on Sunday.