Paul doesn’t find his fortitude or aptitude in his methods, his resume, his experience, or even the testimony of others. Instead, he chooses to believe his qualifications come from God, through Christ, and by the Spirit:
Such is the confidence we have through Christ before God. It is not that we are competent in ourselves to claim anything as coming from ourselves, but our adequacy is from God. — 2 Corinthians 3:4-5 CSB