From chapter 56 to this point - a full ten chapters - the Prophet Isaiah has been, by the Spirit of God, exposing and denouncing the sin of these religious people. They only pay attention to the external rituals of their devotion to God. They are selective hearers of God. They betray Him in the way they have "chosen their own ways"(3) while offering their sacrifices in the temple. Somehow, they have encased their duty to God in the confines of the temple. This, they knew, was God's turf. In the rest of their lives, God says they "chose that in which I did not delight"(4).
Pause and digest that last phrase. God says they "chose that in which I did not delight"(4).How did God know these people were hypocrites? What was the proof their sacrifices in the temple were worthless abominations? Their hearts were prosecuted before God by the way they chose lifestyles that delighted them, rather than those that delighted God. God measures my heart by its consideration of His pleasure, not my own.
Only secular people - people void of the Spirit of God - make their life choices based on self-fulfillment. These people didn't stop, when they were outside the temple, to analyse their choices very deeply. They chose on the basis of their desires alone. They never took the time to assess the things that would be the most pleasing to God.
The Prophet's words in this magnificent text are designed to expose their sin. Its wording is crafted to shock these religious people (O, how wonderfully piercing and unpredictable God's Word is!). And the message is also sweetened with a word of instruction, reproof and hope.