Some people behave really well at church or Sunday school, but afterward, at home or school, they treat their siblings or classmates badly. The Bible talks about this: doing what is right and just pleases God more than any offering or religious ritual.
It's no good singing beautifully at church and then having an ugly fight with your sibling at home. God doesn't just want our Sunday "performance," He wants our whole life, Monday through Sunday.
Think of it this way: it's easy to look good when everyone is watching. What's hard, and what really matters, is being good when no one is watching, treating well the people who are at home with you every day.
Jesus Himself corrected religious people who prayed beautifully but were unjust in real life. He wants our faith to show up in our everyday attitudes, not just in Sunday's pretty words.
God doesn't just want a pretty Sunday, He wants a just whole week.