21 "I hate, I despise your religious festivals;
your assemblies are a stench to me.
22 Even though you bring me burnt offerings and grain offerings,
I will not accept them.
Though you bring choice fellowship offerings,
I will have no regard for them.
23 Away with the noise of your songs!
I will not listen to the music of your harps.
Strong words from God! He despises their religious activities. He hates them doing the very things he had prescribed - festivals, offerings, sacrifices, songs. What had happened that these things had become a "stench" to God?
The people were performing all the religious practices but were corrupt. They hated people who worked for justice or told the truth (5:10). They exploited the poor to increase their own wealth (5:11). They oppressed the innocent, took bribes and did not give justice to the poor (5:12). They were selfish and self-serving, willing to exploit others but turning up at the temple and thinking that going through the motions would mean they were alright with God. That is what God despises.
"impurity of heart vitiated all their works, and this was the reason that God rejected every thing which the Jews thought available for holiness" Calvin
Turning up on a Sunday does not make you right with God. Giving up something for lent does not impress Him. Giving money, reading the Bible, praying, trying hard to be nice to people, supporting charities, not drinking, not smoking, not swearing, or even working for justice does not restore your broken relationship with him if your heart is still turned in on itself. You can do the right things with the wrong motivation and God doesn't accept it. Consider the condition of your heart, not just what you do. You need more than a change in behaviour, you need a heart transplant
I will give you a new heart and put a new spirit in you; I will remove from you your heart of stone and give you a heart of flesh