Remember to whom God was speaking—His people, those with whom He had made the Old Covenant. He was not rejecting their sacrifices or the keeping of the holy days. He was angry that they went through the rituals without the humility to submit to His great moral law in their daily lives.
We have the tendency to think of worship as something we do at a designated time and in a certain place, usually once a week. However, religion and worship in the biblical sense involve all of life. Christianity is a way of life (Acts 9:2; 19:9, 23; 22:4). Worship is the reflection of God living in the person no matter what he may be doing. It is his response to God, his interaction with Him. Thus, the Bible covers every aspect of life within its pages. A person truly interacting with God is worshipping God whether at church, work, play, or home. He will strive to glorify God in every situation.
Obviously, the people of Isaiah 1 were not at one with God, though they religiously observed the commanded activities. For a person to be at one with Him, what he does in every area of life must agree with what he professes by his attendance at a worship service.
How can those who treat their fellows with contempt, then take their greed, anger, revenge, and hatred into church fellowship, say they are displaying God's Spirit? These characteristics are divisive! How can they say they worship God?