Sometimes you say that believing is the same as ‘justifying God’. What does that mean? We do not have to justify God, but He justifies us, or not?

Luther:

Justifying God is agreeing with God in what He says about us. When you agree with somebody, you believe him. This has two sides, because God also says two things about us: acknowledging that it is deserved when God judges us, but also trusting His promise that He wants to acquit us. At the moment we do that, He acquits us and forgives our sins. So our justification of God and God’s justification of us coincide. Actually they are the same.