2 Corinthians 13: check up

The Corinthians were criticizing Paul. Paul wrote:

5-9 Test yourselves to make sure you are solid in the faith. Don’t drift along taking everything for granted. Give yourselves regular checkups. You need firsthand evidence, not mere hearsay, that Jesus Christ is in you. Test it out. If you fail the test, do something about it. I hope the test won’t show that we have failed. But if it comes to that, we’d rather the test showed our failure than yours. We’re rooting for the truth to win out in you. We couldn’t possibly do otherwise.

The test addresses the question: "Is Jesus the Messiah living in me?".

Jesus changes our view of the Lord, of the world, of our values, priorities, of everything. This change of view must be linked to a change in our behavior. Paul wrote:

2 On my second visit I warned that bunch that keeps sinning over and over in the same old ways that when I came back I wouldn’t go easy on them. 

Paul was asking the Corinthians to ask if they belong to Jesus, and if so, it must produce a change of life.
The letter to the Corinthians is not only a defense of Paul, but a pastoral letter to guide the sheep back to Jesus.