Quranic Definitions

2.1

Root ( slh* ): original meaning: to be right, good, honest, upright
as-sālihūn: the righteous, who do good, free from blemish, upright.
Also related: sulh, peace, reconciliation
as-sālihāt, good works
aslaha (iv): to make whole and sound, to amend, to make a reconciliation or peace.

In the Quran the concept of sālih is very much tied to iman (īmān - faith):
One of its most frequent phrases is
{ alladhīna āmanū wa ʿamilū sālihāt:
those who have faith and do good, righteous works -
those are the rightful owners of the Garden; therein they will abide. }
Sura al baqara 2-82 and other verses.

So its is clear that 'good works' sālihāt and 'faith' īmān go hand in hand, the one is strengthened by the other. ( On belief and good deeds, see link )

2.2

Some Quranic Explanations

'Good works' sālihāt are those done only for the Divine Presence or the Countenance of Allah li-wajhi-LLahi.
As described in the Convenant with the people of old (2-83) it includes:

a.) To worship none save Allah,
b.) to be good to parents and, to near kinsmen and to the orphans,
c.) to speak kindly (hasan - good) to everyone,
d.) to perform the daily prayers,
e.) to give the poor-due (zakāt - alms)

2.3

The Way of the Prophets

It was not the way of the Prophets, may Allah grant them peace, to pray to be 'made righteous' or somehow be 'transformed' spiritually, but they all prayed to be among the righteous as-sālihūn. So did Sulaimān 27-19, or Yusuf 12-101, may Allah grant them peace, and of course Muhammad, the Messenger of Allah ﷺ .

So success for the believers lies in preparing ourselves through purification of the nafs and good deeds, then being brought together with the righteous ones as-sālihūn 5-84, in a time when the evil of people is spreading abundantly.

Then Allah takes care of the righteous 7-196, as He does with all of His Prophets (may Allah grant them peace), the masters of His community and His believing servants, when He has chosen them and He places them among the righteous ones, who are the best of mankind especially with Prophet Muhammad ﷺ . See for example 29-9.

2.4

Many quranic concepts highlight the best of charactertraits, as those of the Prophets, may Allah grant them peace, and His awliyā [faithful servants].
So - for example - every righteous one sālih is also truthful sidqun, see 4-69

2.5

The Quranic Verses

{ And remember the time when WE took a covenant from the Children of Israel-
'You shall worship none but ALLAH and be kind to parents and to the kindred and orphans and the poor and speak to men kindly and observe prayer, and pay the Zakāt.'
then you turned away in aversion, except a few of you.} sura al baqara (2) verse 83

{ And whoever obeys Allah and the Apostle, they are with those upon whom Allah has bestowed favors from among the prophets and the truthful and the martyrs and the righteous, and a goodly company are they! } sura an-nisā (4) verse 69.

And what (reason) have we that we should not believe in Allah and in the truth that has come to us, while we earnestly desire that our Lord should cause us to enter with the good people? sura The Table Spread (5) verse 84.

Surely my guardian is Allah, Who revealed the Book, and He befriends the good/ righteous. sura The Heights (7) verse 196.

My Lord! Thou hast given me of the kingdom and taught me of the interpretation of sayings:
Originator of the heavens and the earth! Thou art my guardian in this world and the hereafter; make me die a muslim and join me with the good / righteous. sura Yusuf (12) verse 101.

So he smiled, wondering at her word, and said:
My Lord! grant me that I should be grateful for Thy favor which Thou hast bestowed on me and on my parents, and that I should do good such as Thou art pleased with, and make me enter, by Thy mercy, into Thy servants, the good ones. sura The Spider (27) verse 19.

And (as for) those who believe and do good, We will most surely cause them to enter among the good / righteous. sura The Story (29) verse 9.

And spend out of what We have given you before death comes to one of you, so that he should say:
My Lord! why didst Thou not reprieve me for a little while, so that I should have given alms and been of the doers of good deeds? sura The Hypocrites (63) verse 10.

The Prophet Muhammad ﷺ warned his community of the evil of people's souls, especially unbelievers:

It is narrated from Zainab bint Jahsh, that one day the Messenger of Allah entered upon her in a state of fear and said,
"None has the right to be worshipped but Allah!
Woe to the Arabs from the Great evil that has approached (them).
Today a hole has been opened in the dam of Gog and Magog like this."
The Prophet made a circle with his index finger and thumb.
Zainab bint Jahsh added:
I said, "O Messenger ofAllah's! Shall we be destroyed though there will be righteous people as-sālihūn among us?"
The Prophet said ﷺ, "Yes, if the (number) of evil (persons) al-khabathu increases." Bukhari, bk 88 no. 249

3.1

Root zlm: original meaning: putting in a wrong place
az-zālimūn: the wrong-doers, evil-doers, those who transgress the proper limit (as in sura al baqara 2-229) and encroach upon the right of some other people.

3.2 Some quranic definitions

The wrong-doers are:
a.) Those who transgress Allah's limits (2-229)
b.) Those who invent a lie and attribute it to Allah (3-94)
c.) Those who are disbelievers (2-254)
d.) Those who take the disbelievers for friends (9-23)
e.) Those who judge by other than the law (sharīʿah) of Allah (5-45)

3.3

Those who wrong themselves, their own souls (2-231)

{ wa man yataʿadda hudūda-LLāhi fa-ulāika hum-u zzālimūna:
for whoso transgresses Allah's limits: such are the wrong-doers} sura al baqara 2-229

{ fa mani-ftarā ʿala-LLāhi-l kadhiba min baʿdi dhālika fa-ulāika hum-u zzālimūna:
And whoever shall invent a falshood after that concerning Allah, such will be the wrong-doers} sura āl-imrān 3-94

{ wa-l kāfirūna hum-u zzālimūna:
The disbelievers, they are the wrong-doers} sura al baqara 2-254

{wa man lam yahkum bimā anzala-LLāhu fa-ulāika hum-u zzālimūna:
Whoso judges not by that which Allah has revealed, such are wrong-doers.} sura al māidah 5-45

{ wa man yatawallahum minkum fa-ulāika hum-u zzālimūna:
Whoso of you takes them [the disbelievers] for friends, such are wrong-doers.} sura at-taubah 9-23

3.4

Also: doing wrong to oneself, one's soul, one's (nafs). With this meaning f.ex. in verse:
{wa man yafʿal dhālika faqad zalama nafsahu:
Whoso does that [ to transgress the limits]has wronged his soul.} al baqara 2-231 or f.ex. in al-kahf 18-35, as-saffat 37-113..

In order to avoid this, i.e. to transgress the limits of Allah, and to be able to do what is expressedly stated in the shariyah, the Muslim has to lie in wait ( al mirsād ) for his or her selfishness, never giving it free reign, so that it does not pull him off the right path. RT-663u
As the Prophet Muhammad ﷺ  said when he came back from one of his campaigns:
You have come forth in the best way of coming forth: you have come from the smaller jihad to the geater jihad.' They said: 'And what is the greater jihad?' He replied: 'The striving [mujahadat]* of God's servants against their idle desires.
(*also: spiritual warfare).
Suyuti said that al-Khatib [al-Baghdadi] relates it in his 'History' on the authority of Jabir

4.1 Rootmeaning

Root kfr: original meaning: to cover up, just as a farmer covers the seeds in the earth, also being ungrateful
al-kāfirūn: the disbelievers, the stubborn ungrateful, the deniers of truth.

4.2 Explanation

From the etymogical meaning of the word 'kafara' is covering up, hiding the seeds in the earth (= satara), so the meaning is to hide the truth, further to deny it. Those who refuse to accept the heavenly message brought by a genuin prophet, although they know that it confirms the truth they had from earlier revelations.

The word kāfir also means those who actively deny the truth by attacking Islam or the Muslims by speech, pen or actions etc.

Another meaning of kāfir is of having a corrupt belief, as when distorting the original teaching of the Oneness of Allah.

It also means the one without God, an atheist and acting as such. On the Day of Judgement the kāfir is the one who died as a disbeliever. 2-161
(Ibn Kathir commenting on 2-254, says that there is no greater sin than meeting Allah as a disbeliever. )

He will be in Hell, because he did not have the least amount of faith in the One God.
It does not mean common people going their business, because if they respect Islam and the Muslims, they have to be respected as much, according to the teachings of the Quran and the Sunna. OT

4.3 Some Quranic Definitions

a.) disbelieve in Allah and His messengers,
and also believing in some, while rejecting others 4-150
b.) and in the message which has been sent from God. 41-14
c.) and they judge by someting else than what was revealed from above. 5-44
Ibn Abbas (ra) is reported to have said:
"(Oh Muslims!) Good people you are! The pleasant is for you, the unpleasant is for the people of the book. Whoever denies the decree of Allah is an unbeliever. Whoever accepts it, but does not judge according to it, is a wrongdoer (Zālim)." Zamahs.
d.) the believers have to be protected against. 109-1
e.) because if they but knew - whiles wronging others, they wrong themselves (their souls). 2-254

4.4 Allah Is the Source of All Grace and Goodness

Allah is the source of all grace and goodness, so man owes God being grateful of all bestowed bounties. But the kāfir is not, and worse he even rebells against His Benefactor.

Here are some quranic verses of his reactions to truth and his character:

a.) they do not understand, do not try even, but wondering 50-2
b.) they are astonished at the message 50-3, 38-4
c.) they are ungrateful (original meaning of kfr) 26-19
d.) they are mocking the message 21-36
e.) they deny the message (core meaning of kfr) 29-47
g.) they have defective hearts 22-46
f.) their pride is a consequence of their hardened hearts 7-75
h.) the kāfir's mind described (hearts of stone) 2-74
i.) their hearts are veiled, as if a curtain between it and Revelation 41-5.
j.) a kind of (spiritual ignorance and sin) rust covers their hearts ever more 83-14
k.) that is why they are in a state of deepest (self-)deception 67-20
l.) but at times of distress the unbelievers' nature of kufr is made manifest 42-48
m.) disputing, relying on reason only, not ready for meeting their Lord 32-10
n.) the believers and prophets (as) do not despair, but so do the unbelievers 12-87

4.5 Verses on How the Unbelievers Are (Re-)Acting

a.) As above mentioned they always act in pride and deny the message 6-29
b.) they disbelieve in the Resurrection taking to reason instead of the Revelation 6-29 and 32-10
c.) they disbelieve in the signs (i.e. favours) / verses of Allah and deny them 3-70
d.) they doubt anything which is not what they call "reasonable", f ex that a normal man was given prophethood. 54-24
e.) there are against the truth of the Oneness of Allah (tauhīd) 38-5
f.) they would deny the mentionng of the All-Beneficient (ar-RaHmān) 21-36 see above
g.) there are arrogant 21-4
h.) their contentiousness and their asking the Prophet ﷺ embarrassing questions 40-4
i.) what is worse the debar other people from the right path 11-19

4.6 Some Quranic Verses on the Disbelievers' Unhappily Ending

a.) they will not be not successful, nor happy, not in bliss 28-82
b.) Judgement-day will be a bad day for the unbelievers, a stern punishment 42-26
c.) because they were used to call the divine truth being unreal or 'magic' 43-30
d.) they will regret not having used their intellect 67-10

4.7 Quranic Advice

a.) Do not take them for friends, as long as they prefer disbelief for belief 9-23
b.) It is a waste of time to try to guide them 27-80