The Quran describes different types of people as follows:
People of the Book (Ahl E-Kitaab)
Disbeliever / Rejector (Kafir)
They are polytheists (people who worship more than one God). Mushrikeen comes from the Arabic word “shirk” which, in general, means “to share”. With respect to the Quran, “shirk” means sharing others as partners besides God.
They are monotheists (people who submit to one, true God). They are not necessarily Believers as true belief may not have entered into their hearts yet. Being a submitter is a prerequisite to being a believer.
The desert Arabs say, “We believe!” Say, “Do not say you believe; but only say, ‘We have submitted our wills to God (as Submitters / Muslims),’ as belief has not yet entered your hearts. But if you obey God and His Messenger, He will not deprive you anything of your deeds: for God is Oft-Forgiving, Most Merciful.” (49:14)
They believe in God, His angels, His books, and His messengers and don’t differentiate between any of His messengers.
“The messenger believes in that which has been revealed to him from his Lord and (so do) believers. Each one believes in God and His angels and His scriptures and His messengers – We make no distinction between any of His messengers – and they say: We hear, and we obey. …. ” (2:285)
Prophet Moses was a Submitter (Muslim) and after seeing a sign from God, he became a Believer (Mu’min)
“And when Moses came to the place appointed by Us and his Lord spoke to him, He said: “O my Lord! show (Yourself) to me, that I may look upon You.” God said: “By no means can you see Me; But look upon the mount; if it remains in its place, then shall you see Me.” But when his Lord manifested His glory on the Mount, He made it crumble to dust. And Moses fell down in a swoon / unconscious. When he recovered his senses he said: “Glory be to You! to You I turn in repentance and I am the first to believe.” (7:143)
Disbeliever / Rejector (Kafir)
Disbelievers (Kafirs) are people who, after understanding the message of truth (worship the one, true God), deny the truth. They intentionally disbelieve in the one, true God. Kafir comes from the Arabic word “kufr” which means to hide. Kafirs hide the truth and refuse to accept it. They will go to Hell.
“As for the Disbelievers, Whether you warn them or you warn them not it is all one for them; they believe not. God has sealed their hearing and their hearts, and on their eyes there is a covering. Theirs will be an awful doom” (2:6-7)
Even if Prophet Muhammad asks God to forgive disbelievers, God will not forgive them. They are walking dead because they’re just waiting to go to Hell.
“Ask forgiveness for them (O Muhammad), or ask not forgiveness for them; though you ask forgiveness for them seventy times God will not forgive them. That is because they disbelieved in God and His messenger, and God guides not wrongdoing folk” (9:80)
Idolaters (Mushriks) are not automatically Disbelievers (Kafirs)
In verse 9:6, the idolaters were not branded as disbelievers because they did not know they were wrong. If an idolater wanted protection, Prophet Muhammad was instructed to protect them so that they could learn about Submission (Islam) and become a Submitter (Muslim). An idolater only becomes a disbeliever if they’ve seen the truth about the one and only God yet they still refuse to believe.
“And if anyone of the idolaters seeks thy protection (O Muhammad), then protect him so that he may hear the Word of God, and afterward convey him to his place of safety. That is because they are a folk who do not know” (9:6)
The People of the Book (Ahl e-Kitaab)
They are people who received the former scriptures (Torah, Injil). They are Jews and Christians. Jews and Christians who submit to one, true God are Submitters (Muslim).
“Not all of them are alike: Of the People of the Book are a portion that stand: They rehearse the verses of God all night long, and they prostrate themselves in adoration. They believe in God and the Last Day; they enjoin what is right, and forbid what is wrong; and they hasten in good deeds: They are in the ranks of the righteous” (3:113-114)
Can Believers (Mu’mins) be friends / allies with Jews and Christians?
According to verse 5:51, God instructs believers to not be allied with the Jews and Christians.
O you who have believed, do not take the Jews and the Christians as allies. They are [in fact] allies of one another. And whoever is an ally to them among you – then indeed, he is [one] of them. Indeed, God guides not the wrongdoing people.
However, when read in context, we see from verse 5:57 that the prohibition is only against being friends / allies with Jews and Christians who ridicule Islam.
O you who have believed, take not those who have taken your religion in ridicule and amusement among the ones who were given the Scripture before you nor the disbelievers as allies. And fear God, if you should [truly] be believers.
At the time of Prophet Muhammad, some People of the Book (Jews and Christians) were Believers (Mu’mins) because they believed in the Quran and Prophet Muhammad.
“And there are, certainly, among the People of the Book (Arabic: Ahli-l-kitabi), those who believe in God, in the revelation to you, and in the revelation to them, bowing in humility to God: They will not sell the Signs of God for a miserable gain! For them is a reward with their Lord, and God is swift in account” (3:199)
Most people are Disbelievers (Kafirs)
As of 2018, despite Islam being the 2nd largest religion after Christianity and being projected to become the largest religion some time between 2051 and 3000, verse 12:103 indicates that most people may still call themselves Muslims (Submitters) or Believers (Mu’mins) when in reality they are actually disbelievers (kafirs).
And most of the people, although you strive [for it], are not believers. (12:103)
It is possible, however, that verse 12:103 may mean that most people during the time of prophet Muhammad were disbelievers, rather than most people throughout life in this world. Nevertheless, verse 12:103 is in agreement with 17:62 whereby Satan says he will destroy all but a few of among mankind, even though 17:62 simply indicates what Satan said and doesn’t necessarily mean what he said will actually happen.
When the hypocrites come to you, [O Muhammad], they say, “We testify that you are the Messenger of God.” And God knows that you are His Messenger, and God testifies that the hypocrites are liars. (63:1)
The hypocrites (Munafiqs) will be in the lowest depths of Hell