Bring no more vain offerings;
incense is an abomination to me.
New moon and Sabbath and the calling of convocations—
I cannot endure iniquity and solemn assembly.
God had enough of Judah's pretense. He didn't want people showing up at His temple, going through the motions without really being serious about their sin or His holiness. You can't make God your lucky ritual and live as you want in your iniquity. Sin is serious business. Dead. Serious. Business.
Our sin separates us from God. Sin costs us fellowship with God. Sin incurs God's wrath. Isaiah was a prophet of God sent to Judah to call the people of God back to taking Him seriously. And to do so meant they had to deal with their sin seriously. Repentance needed to precede all their activities or else true worship could not happen. It just added to their offense. God did not commend their unholy, flippant, religious ritual. He wanted their repentant, heartfelt worship.
Judgment followed the cavalier dismissal of sin. Death is always the end of sin. That is the universal constant. Sin entered the world and death is always with it. Only repentance and faith in God's provision to atone sin will remedy this. Judah needed to repent and in turning to God with sincere faith, offer sacrifices.
Since Jesus died for all sin, such sacrifice is no longer needed on our part. But seriousness about sin, repentance, faith, and obedience to the gospel are needed. When I go through religious motions, whether in Sunday assembly, or in my workday devotions, without a seriousness about my own wickedness, I have done wrong and must repent.