KJV translation of Greek adokimos, referring to battle-testing of soldiers, qualifications for office, or testing of metals to make sure they are genuine. Paul used his own example of personal discipline to ensure that his preaching proved true in life as a call to others to do the same (1 Corinthians 9:27). He did not want to be cast away as impure metal or disqualified as an unworthy soldier or candidate. Paul played on the words dokimos, “qualified,” and adokimos, “disqualified,” in
2 Corinthians 13:5-7. The Corinthians demanded a test or proof that Christ spoke through him (2 Corinthians 13:3). Paul turned the argument on them, saying they needed to prove themselves that they had not failed the test of Christ and become reprobates. He hoped the Corinthians would recognize in Paul's life that he had not failed the test and was thus not a reprobate. He prayed the Corinthians would not do wrong, not to prove himself qualified but so that the Corinthians would do what was right even if Paul proved to be unqualified.
Paul warned Timothy of evil persons in evil times with people resisting the truth, having corrupt minds, and being unqualified in the faith (2 Timothy 3:8). Similarly, he wrote Titus of persons professing to know God but unqualified in good works (Titus 1:16). With similar purpose, Hebrews compares people to ground which bears thorns and briers and is thus unqualified and fails to pass the test (Hebrews 6:8).