In the first reading for Sunday (Deuteronomy), we're listening in on a larger conversation where the most famous relationship in the Bible – God and His people – is being sorted out. What does it mean for them to be God’s people? What does it mean for Him to be their God?
You wouldn’t get married without a lot of deep conversations with your intended. You’re not stupid. Neither is God. Since this is going to be a relationship for the ages, they've got a lot to talk about.
And this is the backstory for Sunday's Gospel (Mark). All of the things that God's people do, their special observances, etc., all of it flows from this relationship. It's supposed be offerings to God made in love. And to call them back to God during the busyness of life.
But it's not, not anymore. They've lost touch with the purpose for what they're doing. And with the One they're doing it for.
Which is why Jesus is calling them out.