"Very well then, with foreign lips and strange tongues God will speak to this people, to whom he said, 'This is the resting place, let the weary rest'; and, 'This is the place of repose' - but they would not listen." NIV translation
Isaiah's answer to the religious leaders of Jerusalem who mocked him was to prophesy that they would have to learn the hard way. He basically told them that he was one of them but since they would not listen to him, then, God would send foreigners to conquer the city.
He went on to remind them that God had given them the land that they now occupied and that He had promised to care for them. They had abandoned that relationship and grabbed onto religious practices that were performed insincerely and merely as a duty. God was to be their ultimate ruler but they had decided that each man could rule their own lives. That is pretty much the way that things are today and it will get even worse in the final days.
"So then, the word of the Lord to them will become: Do this, do that; a little here, a little there - so that as they go they will fall backward; they will be injured and snared and captured." NIV translation
The leaders refused to repent of their ways and to acknowledge that they would not be saved by half-hearted religious rituals. Therefore, Isaiah says that God will bring foreigners against them to capture them and take them into slavery.
The idea of falling backward brings to mind the picture of walking along and unexpectedly slipping and losing your footing. This is what was happening with Judah and its religious leaders as they were confident that they were doing just enough to stay on good terms with God. The same thing is happening in our world today as many people go through the motions of religion hoping that they have done just enough to satisfy the requirements of God. Like the rulers of Jerusalem, they will be surprised when God sends the judgment and they are treated in the same manner as all the others that rejected His grace.