The writer tells us Jesus’ sacrifice is greater than those described in the Old Testament. The Old Testament sacrifices had to be done on an ongoing basis; Jesus was only sacrificed once. Had the Old Testament sacrifices been sufficient, they would not have needed to continue. In this way, the Old Testament sacrifices made it obvious they were to be replaced by something greater. The Jews who are turning away from Christ and back to being Jews were turning their backs on the greater sacrifice, Jesus, and on their fellow Christians. They are warned not to do that.
It isn’t easy to follow Jesus. We may be tempted to walk away from God or the church. Walking away from Jesus is the biggest mistake a person can make, but walking away from the church is close to it. The church is supposed to be a gathering of people trying to follow Jesus as best they can. We need each other to keep going, especially when it’s hard.