The Old Testament quotes were about God talking to the Israelites when they were about to take the Promised Land. You really can't apply them to Christians today.

In Hebrews the writer is writing at a time when tens of thousands of Christians were being savagely murdered every year. Some burned to death or crucified. Others killed by wild animals in the arena along with their terrified children. Christian women were being systematically raped and murdered. So despite the incredible suffering of the early church at that time he still says that God will neither fail us nor forsake us. We know that no matter what happens to us that God will take us safely to heaven after we die. So when it comes to our eternal souls we know that God will never fail us nor forsake us.