“Test yourselves to see if you are in the faith; examine yourselves! Or do you not recognize this about yourselves, that Jesus Christ is in you—unless indeed you fail the test?” 2 Corinthians 13:5
Did the Apostle Paul have and preach a Biblical Gospel? “Of course he did!”. And did the Church at Corinth hear this Gospel, respond to it and receive the Holy Spirit, so much so, that Paul says they were not lacking in any gift? “Yes, they did!” So then, why would Paul exhort them in this manner? Why would he admonish them to look within? Because we are still Sinners and still prone to deception!
Remember, there are Many that think they will be welcomed into Heaven on the Last Day but instead be turned away. No doubt, they did not Test Themselves! May God give us Courage and Wisdom to check our own Hearts!!!