The word adokimos is rendered as reprobate, castaway, or rejected. It represents the condition of sin itself, but not a particular sinful action. Those who do not want to retain a knowledge of God in their thoughts are given over to a reprobate mind (Rom. 1:28). This is the kind of mind that refuses to give thanks, and which refuses to honor God as God. Not surprisingly, this kind of mind is firmly established among false teachers. Jannes and Jambres opposed Moses, and the in the same way men of reprobate mind mislead silly women, corrupting the faith (2 Ti. 3:8). False teachers will claim to know God, but by their works they deny Him. They are abominable, disobedient, and with regard to good works they are rejected (Tit. 1:16).
But the elect do not glide into glory on a conveyor belt — we are to pay close attention to the question. We have to be careful not to use the false standards of the emotional pietist, but this does not mean there are no standards to apply. The apostle Paul was careful to keep himself under a strict spiritual regimen, lest he find himself a castaway, a reject, even though he had preached the gospel to others (1 Cor. 9:27). Christians should be diligent to consider their own true spiritual condition, testing themselves. Jesus Christ is in the Christian, unless that Christian turns out to be a reprobate (2 Cor. 13:5). Paul also assumes that the Corinthians know that he, the apostle, and his company are not reprobate (v. 6). But if the Corinthians simply did what was right, Paul was willing to look like a reprobate (v. 7). In all this, Paul is assuming the possibility of self-deception, and the need for a scriptural heart-check. Land that is blessed by God, but which produces thorns and thistles in response, is rejected land (Heb. 6:8).