Proverbs 27:17, 27:19

People’s faces in front of God are not those of the physical body, and what people look like in front of God are not their physical shapes. Instead, the summaries of the intents and attitudes of the heart and all the thoughts and understandings of the mind, appear as a spiritual face in front of the LORD. That is what people look like in front of God.

The more a person’s heart and mind resemble God (that is, memorize the Word of God, all of it, and follow the instructions), the more the person’s face (in the Spirit world) is transformed to resembles God, and the more the person is familiar to God.

The more a person’s heart and mind have sin and impure thoughts/feelings in them, the more the person’s face (in the Spirit world) is transformed to resemble those of devils, and the more acquainted the person is towards evil thoughts, and the heart/mind of the Devil himself.

Whoever memorizes all of the Word of God (from Genesis to Revelation, and it’s possible to accomplish that with a few years), acquires much resemblance with God.
Of course, having remembered, you must follow the instructions. Then you are clean-hearted and same-minded with the LORD, and you resemble God as much as possible (and that is your best life possible, to memorize all the Bible and follow it).

(note: if you memorize in English, you must only use 1611 edition, KJV, do not use any other version, even NKJV is perverted).

Iron sharpeneth iron; so a man sharpeneth the countenance of his friend.
Proverbs 27:17
As in water face answereth to face, so the heart of man to man.
Proverbs 27:19

The Bible is a holy book full of spirits, and the world of spirits (of Holy Spirit and spirits of saintly men in heaven) is the greater world, this material world is the lesser world (exceedingly less, like a small mound of dirt compared to a whole universe).

God is a Spirit (John 4:24), and when God thinks, He first thinks in Spiritual world, then He thinks about this smaller world of material things. So for nearly every part of the Word of God, whenever there is talking about this material world, there must (nearly always) be a ever greater meaning in the spiritual world. Any reference to this material world is more immediately understood, but it’s also the lesser meaning. For example, “eat” is immediately understood as referring to intaking food in this material world, but actually, God primarily thinks of “eat” as intaking holy knowledge and wisdom into the heart/mind and Spirit. And nearly all the “eat” in the Word of God does actually refer to that, it just happens that God uses the situations of this material world as metaphors to describe the spiritual world.

So that “water” mostly refers to spirit, in the greater world of spirits, where there are streams of spirit instead of streams of water. We know that, because Christ said that He is the living water (John 4:10, 4:14).

Thus, in Genesis 1:2, “the Spirit of God moved upon the face of the waters” = “the Spirit of God moved upon the surface of many spirits”. And in Revelations 13, the anti-Christ beast rise out of oceans; the oceans are described in Revelations 17:15 as many unbelieving spirits among the many peoples, nations, languages, and kings.

Continuing with the commentary about the Proverbs 27:17, 27:19,

We already know from the Word of God that “water” refers to spirit, so “in water” = in the spiritual realm.

Thus, we also know that as far as God (Yehovah) is concerned, what people look like is not the face of their physical body, nor their physical shape, but instead the appearance of their heart/mind in the spiritual realm, that is “the face” or “the countenance” of the person, as God sees it everyday.

For God is not a man, that His ways (perspectives and functioning) would resemble those of humanity.

Thus, the heart and the mind of a man, if the man lives godly (with a clean heart, and the mind of Christ, with much of the Word of God memorized), sharpens the heart and mind of his friend.
What does sharpening refers to?  Probably it refers to the spiritual world where the face of his friend is getting transformed to more resemble God.  This means the heart and mind is getting transformed to more resemble how God thinks and how God feels about everything.

How does the heart/mind of a man affect that of his friend?
Whenever you spend time with a person or accept communication from a person, you are getting affected or transformed by that person.

When you spend time or accept communication from people, then you are communing (yes, same as taking communion) your heart and mind with that person.   In communion, you commune your heart and mind with the heart/mind of the LORD.   But you do the same with people you spend time (or conversation) with.

If you spend much time with people of filthy mind (who will speak filthy words, steer your mind towards filthy thoughts, even add filth to any Bible they talk about and demonize the Word of God), then your mind gets “marinated” with the filth of their minds,  and your mind becomes more like theirs.   Your heart also becomes more and more filthy, becoming more like theirs.   And since actions come from the intents and attitudes of heart/mind, you will also start talking and behaving more like them.

For this reason, God says to cut off communications when you start noticing that people’s mind are filthy, or that they are not sincere towards God, or that they think evil.   You are to avoid getting influenced by such people.

Go from the presence of a foolish man, when thou perceive not in him the lips of knowledge.  Proverbs 14:7
Forsake the foolish, and live; and go in the way of understanding.  Proverbs 9:6
He that walks with wise men shall be wise: but a companion of fools shall be destroyed [by God].   Proverbs 13:20

He that hides hatred with lying lips, and he that utters a slander, is a fool.  Proverbs 10:18
….. the folly of fools is deceit  Proverbs 14:8
Fools make a mock at sin …..  Proverbs 14:9
He that is soon angry deals foolishly Proverbs 14:17
A fool has no delight in understanding, but that his heart may discover itself.  Proverbs 18:2  
(All psychologists are fools, and you are to cut off interaction with them)

He that trusts in his own heart is a fool …..
Proverbs 28:26
(those who take any desire of their heart, and call it a “need”, and then prioritize their life to satisfy “needs”, they are fools.)

Speak not in the ears of a fool, for he will despise the wisdom of thy words
Proverbs 23:9
(don’t waste time rebuking people who are obviously foolish, rather spend time instructing people who are willing to learn, seeing that each person has limited time)

The thought of foolishness is sin: and the scorner is an abomination to men.
Proverbs 24:9
(don’t interact with fools, or look at the words/images produced by them, lest your mind become like theirs, and you sin)

It is as sport to a fool to do mischief: but a man of understanding hath wisdom.  Proverbs 10:23
He that devises to do evil shall be called a mischievous person. Proverbs 24:8
For their hearts studie destruction, and their lips talk of mischief.  Proverbs 24:2

Instead, you are to commune your heart and your mind with those who are of pure heart/mind, those who think holy thoughts.    First of all, such a person is God.
If most of your intaking of words consists of viewing/hearing corrupt communications (words, images, sounds) made by filthy minds and wicked hearts, then your heart/mind will get “marinated” into becoming filthy and wicked.
If most of your intaking of words consists of viewing/hearing godly and clean messages from God or godly teachers (there are few in these times, most of them are from Puritan times and medieval times and Early Church),  then your heart/mind will also become like God’s heart/mind.

God speaks in the Scriptures (Genesis to Revelations). 
Make sure your own language-use is not corrupt.  For instance, if you read “fruits”, you have to think “the good result of many months or years of spiritual cultivations”.

You need to memorize all Scriptures.

Godly men speak in their sermons and written works.
(the very best to read is Luther and Jerome.  Then there is also Manton and Augustine, who have rare mistakes.
Until you finish reading all of Luther, Jerome, Manton and Augustine, stay away from Jonathan Edwards and John Wesley, because they are from after the Puritan era, they have greater imperfections and are sometimes spiritually malformed.)

The more current preachers are worth little compared to Luther, Jerome, Manton and Augustine.   Furthermore, beware of any modern book in “Christianity” section (written by people still on earth), because they are nearly all of them polluted/corroded by Psychology, which is anti-Christ enemy of the Spirit of Christ. 

Psychology is based on pleasing personal desires (propagandized as “needs”), with your own self as the center of the universe.   It’s earthly, sensual, demonic (James 3:15)
Spirit of Christ is based on denying the self, and pleasing God, God’s needs, which are for you to (memorizing the entire Bible) know truth, justice, mercy, humility in front of God.   God is the center of the universe.

Some examples of Psychology-based “preachers” and “counselors” (who are blind leading the blind):
Cloud&Townsend (“New Life Ministries”), Gary Smalley, Joel Osteen, Rick Warren, John Eldredge, and any other based on experience/emotions rather than Scripture and correct doctrine.

Going back to the main topic:

Your face in front of God, is the quality of your heart and mind, the cleanness of your feelings and your thoughts in front of God.   And also how much you know the Word of God (know = memorize and understand, and follow the instructions).   These added up is your face in front of God.

The following are some parts of the Word of God further showing that when God mentions “countenance”, He primarily refers to the quality and content of the heart, and/or the mind.   Only a tidbit refers to the physical face of the person.

The wicked, through the pride of his countenance, will not seek after God: God is not in all his thoughts.
Psalm 10:4

The shew of their countenance doth witness against them; and they declare their sin as Sodom, they hide it not. Woe unto their soul! for they have rewarded evil unto themselves.
Isaiah (Isa-YAH) 3:9

(this, about people whose heart/mind constantly think of sin, and commit sin openly

Then the king’s countenance was changed, and his thoughts troubled him, so that the joints of his loins were loosed, and his knees smote one against another.
Daniel (Dan-EL) 5:6
Then was king Belshazzar greatly troubled, and his countenance was changed in him
Daniel 5:9
Hitherto is the end of the matter. As for me Daniel, my cogitations much troubled me, and my countenance changed in me: but I kept the matter in my heart.
Daniel 7:28

The LORD lift up his countenance upon thee, and give thee peace.
Numbers 6:26