Continually Confessing (2 Corinthians 13:5)

We can fall into a trap and think that once we believe the gospel we know the gospel. However, this is not what we find in the Corinthian church. When Paul writes his second letter to the Corinthian church they forgot the gospel. Paul exhorts them to examine themselves to see if they are in the faith. This is pretty strong that we are to examine ourselves to see if we are in the faith. How do we know if we are in the faith? Can we have assurance that we are in Christ? What is Paul's point and what is this standard?