This psalm moves suddenly from thanksgiving to a cry for help amid suffering. Life is like that. But David grabs hold of a truth that keeps him from sinking. The core sin is to forget that God is God and that we are not. And this is justice—those who forget God will be forgotten, but those who remember God will be remembered forever (Isaiah 56:5). Christians know of one who remembered God yet was completely forsaken (Matthew 27:46). But because Jesus died in our place, we can be even surer than David that God will always be there for us.
