Ultimately, everything depends on discernment - by which I mean the capacity to evaluate and distinguish-between Good and evil.
Yet discernment cannot be done on objective grounds, but must be done by inference.
And our ability to discern is fallible - we can be fooled, we can fool ourselves, we can mistake expediency for Goodness (or, indeed, suffering for Goodness)... there is no reliable and valid method of discernment.
As an example of the situation we find ourselves in, I will use the discernment between angels and demons (who can simulate or impersonate angels).
1. We cannot do without angels - by which I mean that they influence us and the world, and if we choose to ignore this influence then this is a self-blinding against a vital aspect of reality.
Since we are not meant to ignore (=deny the reality or significance of) angels - we must discern them from demons.
Ignoring the whole issue is therefore not a 'safe option' - we need to discern.
2. It is difficult to discern angels from demons. There are no objective, measurable, publicly-agreed criteria (e.g. demons may appear and beautiful and feign to be Good; angels may be tough or even harsh in their actions or advice, because they understand more and see further).
3. Although we cannot discern objectively or reliably; because we need to discern, we can assume that we can discern - since God is both our loving Father and the creator.
4. How then are we 'supposed to' discern angels from demons if both our intellect (mind) and instincts (gut-feelings) are so prone to error as to be all-but useless?
The answer is by The Discernment of the Heart.
This is something that in some ways we do naturally - but we tend to over-ride it with intellect and/ or instincts; and we also need to learn to distinguish and comprehend it.
The discernment of the Heart is possible because it was built-into us by God, as our ultimate and bottom-line guidance. As such it is immune to distortions of logic' or 'evidence' and to emotional manipulations - it is reliable and true; but the 'flip side' is that the signal is so simple and pure that it is hard to justify, indeed sometimes it is hard even to notice the evaluation of the Heart among the noise and distraction of the world and our minds and bodies (and the deceptions of demons!)...
(Because it has no method or measure, the discernment of the Heart is also easy to deny in a materialist society such as this one.)
5. The situation is: we must discern, we can discern, but we will err in this - not by error of discernment, but by ignoring, over-riding, or being unable or unwilling to detect this discernment of the Heart.
6. Therefore we must discern, we will err; and therefore we must be ready and willing to repent.
7. This, then, is the Human Condition. This is our destiny and path and how we are meant to live.
The 'set-up' is that:
1. we must discern
2. we will err
3. we need to repent
This is - broadly - how we need to approach matters such as angels, demons, religious experiences, signs of the Holy Ghost, the evaluations of truth and holiness, the validity of theories (including theologies), books (including scriptures), institutions (including churches) and persons (including leaders and teachers).