ADOKIMOS     "Reprobate" or "Castaway"
(not approved, rejected as not genuine, as in counterfeit coins)

Apostereo

We see in 2 Cor. 13:5-7, that if one is not of the faith he is reprobate (adokimos) .  To test if one is of the faith or not is the same thing as testing if one is reprobate. Or, as the NAS and NIV translate, "fail the test".

Adokimos is used five more times in the NT. Four times the meaning is that the word of God was not engaged (it didn't 'take') in the heart. Those verses are Romans 1:28, 2Tim. 3:8, Titus 1:16 and Heb. 6:8.

That is why we see in 2 Cor 13:8 Paul's one verse summary of what it means to be "of the faith" (not adokimos):

"For we can do nothing against the truth, but for the truth."

That is also a one verse summary of 1 John where we see over and over that those born of the word of God will be giving evidence of that seed of God which remains in them.

In the remaining usage of adokimos, 1 Cor 9:27, Paul says he is constantly on the look out lest he be fake-engaging God's word in the flesh and not in the Spirit; doing so would
'disqualify' him for heavenly crowns and his works would be consumed as wood, hay and stubble at the Day of Christ.
Apostereo