Jesus told them he would be crucified, but apparently he gave very little teaching about how his death relates to our forgiveness and acceptance in the family of God. Reflection on the doctrine of the atonement came later. To understand the cross we have to know the cost of loving. Anybody who loves deeply will get hurt. Parents hurt their children.
Children hurt their parents.
Husbands hurt their wives, and wives hurt their husbands. And often the one who hurts us does not even know it. But the more we love, the more we are likely to get hurt. Those who love and serve others experience ingratitude, being taken advantage of, misunderstanding, and often being hated. Now if we project our little human experiences of loving and being hurt to infinity, we get some idea of how much God is hurt in loving us.