Discernment is the ability to know the right thing to do - in fact to know the right thing and distinguish it from wrong things: especially wrong things in disguise. Discernment can be imagined as our guidance-system.
Many traditional Christian denominations and churches are set-up on the basis that we will be taught the right things, and given good advice, by those in authority - so the guidance system comes from priest and pastors - and sometimes from Kings and judges too.
But the modern world is characterized by the fact that it is precisely those in authority who are most deeply wicked; who are teaching the wrong things, giving bad advice (advice to be bad), and punishing good behaviour - they are not just muddling-up the right and wrong things, but deliberately reversing the labels...
And the modern world is huge, noisy, distracting, and the good and the right are mixed with, and almost lost in, much larger quantities of their opposites.
So the modern condition is characterized by a greater-than-ever need for a guidance system; while at the same time traditional guidance systems have been subverted and sabotaged.
One single source of guidance is inadequate, because all guidance is general and imprecise, and there will always be ambiguities and uncertainties when specific situation are tested against general rules.
The inadequacy, insufficiency, distortedness of trying to be guided wholly by list of laws and rules should be obvious; and furthermore it is anti-Christian to suppose that our job in Life is merely to obey - it is not.
We are each of us persons - hence unique; and our main job in Life (having accepted the reality and Goodness of God) it is to chisel-out our our own 'salvation', that is to follow the path of theosis, sanctification, spiritual progression or deification - towards the goal of becoming Sons of God and able and worthy of living with and communicating with God - person with person.
So in pursuing this complex path through a complex world; we need a multi-level, multi-step guidance system - and, luckily, we have one:
Knowing God's nature
If we have a reasonably clear understanding of the kind of person God is; what he is like and what he wants from us, and the attributes he most values in us (love above all, intelligence, strength, creativity etc) - then this provides the general background and basis for our guidance.
In particular, God being our loving Father and we his children whom he wants to raise to be Sons of God and 'divine friends' - this understanding rules out a lot of false, deceptive and just-plain-mistaken teachings.
Then we communicate with God by prayer.
Knowing God is in us
If God was out there, and could only reach us by the normal methods of communication - then our situation would be hopeless.
But God is within us - he has planted a glowing coal of His divine nature in each person.
Once we know this glowing coal of divinity is in each of us, we can learn to feel and abide by its guidance - with attention to its promptings, through quiet contemplation and listening, by sensitivity to intuition sent from it, by meditation.
Our free will, agency, autonomy operating on personal revelation
Personal revelation can be grounded in firstly the faith that God is a real person and also within us, and knowing the general direction of God's wishes and hopes for us; and secondly recognizing our own radical autonomy to decide what to do about this.
We know something of God's nature, and we feel something of God in us - and then we are able, and indeed we must and do choose either to recognize God's will, and ally ourselves with Him in His hope for us and for the world - or we oppose it.
Synchronicity is one important but neglected mode of personal revelation.
(Is you, here, now, reading this an example of synchronicity?)
Meaningful, enlightening, and Good coincidences occur and show us the proper or best path life lays before us through the world.
Representing fate, destiny - all these in a wholesome sense because always entailing and requiring our personal choice, decision, whether or not to follow this path.
Good choices are rewarded: when synchronicity is recognized, and when the decision is good, then Life becomes enhanced, infused with a kind of 'magic'.
The channels of communication between ourselves and God open-up, and our path becomes clearer both before and behind us - as if lit by an inner glow.
Personal revelation (God's multi-modal communications to us, personally, for our personal guidance and benefits) takes us from the general to the specific. Takes us from the general knowledge of the kind of thing we must do, and must not do; to specific personal guidance about what exactly we must do, or not do.
We have a superb, flexible, infallible, guidance system!
But, typically, Life is trial and error, and discernment works by zig-zags.
We are engaged with Life, and we make mistakes and we sin due to ignorance, weakness, short-termism - and deliberately too, as an act of defiance.
But as we err and sin - we also discern our sins and errors, and can repent them, and repudiate them. Almost certainly, we will not be able altogether to stop making mistakes and stop yielding to temptations - but that is not what is asked of us nor what we are equipped to do.
We are equipped to try our best but make mistakes, to try our best but yield to sins - and then for discernment to make clear to us what we have done, so that we can (and do) know what was good and what we must repent.
So, our guidance system will lead us to good choices, but it is not perfect and neither is our will; and then the guidance systems will discern what has happened, and reveal our situation so we can do the right thing.
In sum our guidance system - like all sophisticated and really-useful guidance systems - has a predictive element which tells us what to aim at and do; but also a retrospective and corrective element: which alerts us when we are aiming off-course or have travelled down a wrong path and points the direction to get back on course or states the need to stop, turn around and retrace our steps.
(When I say corrected, I do not mean reversible - because once done mistake and sins, like everything else, are permanent. But by the atonement of Jesus Christ, repentance negates any and all sin. Thus - the major function of discernment is not avoidance but repentance.)
Our guidance system is therefore superb and infallible when taken as a whole and understanding its proper function - including both the predictive but also the corrective elements as back-up.
We will make mistakes because the predictive element is imperfect, and we will make mistakes because of our imperfect and sinful nature and the difficulties of our situation - but these mistakes will always be detectable and correctable because the back-up element is an intrinsic part of the guidance system.