At the beginning of Islam, the number of literates in the whole Mecca was about 17 people (1). In those times, it was even considered as a shame to be able to read or write (2). In the ancient times, people mostly weren’t able to read and literacy was the exclusive privilege of the royal families or rich people. However, in the Arabia, even the royal families couldn’t read or write. Once, Prophet Muhammad (PBUH & HP) wrote a letter to “Bakr ibn Va’il”, a large Arab tribe, and enjoined them to believe in Islam; but when they received the letter, they weren’t even able to read it (3).
In spite of these shameful circumstances, Allah al-mighty sent His most important apostle to the people with only a book as his miracle. The first verses of the Quran, revealed to the Prophet (PUBH & HP), are about reading:
Read in the Name of your Lord who created; (1) created man from a clinging mass. (2) Read, and your Lord is the most generous, (3) who taught by the pen, (4) taught man what he did not know. (5) [96:1-5]

Knowledge and Literacy in Islam

Muslims differentiate between the era before the Messenger (PBUH & HP) and the era after him based on peoples’ knowledge. We call the time before the rise of Islam “The Ignorance era”. 
The holy Prophet (PBUH & HP) taught people that knowledge is the criterion that sets them apart   . The Holy Quran says:
Say, ‘Are those who know equal to those who do not know?’ Only those who possess intellect take admonition (39:9)
About learning, the Holy Messenger (PBUH) says:
 Seek knowledge even if you have to go to China to find it. (4)
Imam Ali (AS) also highlights the Importance of knowledge in his speech with one of his companions:
O Komeil! Knowledge is better than wealth, because knowledge protects you. But for wealth, you have to protect it. Wealth and money get less when you spend them, but by using your knowledge or by teaching it to others you even get more knowledgeable … O Komeil! The ones who love wealth excessively, are like dead people (spiritually) even if they are alive (Physically). However, the spirit of knowledgeable ones will always be alive in the hearts of people even if their bodies are dead. (5)
The Holy Messenger (PBUH & HP) even considered learning as “Vajib” which means that learning is an obligation for every Muslim:
Seeking knowledge is an obligation for every Muslim, men and women (6)
One of the clearest signs that shows the Prophet (PBUH & HP) wanted everybody to be literate and knowledgeable is that he ordered the slaves after the Badr war to teach the children of Medina to read and write and in return, the slaves would be free. (7)

Keeping the Knowledge Safe

The Holy Quran says:
By the pen and what they write (68:1)
We all know that without writing, we weren’t able to communicate our thoughts and ideas with each other and we couldn’t keep them for future generations. In this regard, the Holy Prophet (PBUH & HP) says:
Keep your knowledge safe.
Some people asked:
How can we do that?
The Prophet (PBUH & HP) replied:
By writing it! (8)
Moreover, Imam Sadiq (AS) said to one of his pupils, Mufazzal ibn Umar:
Write and spread your knowledge among the people and if you were going to die, leave your writings for your children because there will be a chaos in the future and in those times, people will not find any peace except in their books (9)
He also said:
Keep your writings because you will need them in the future (10)
Imam Hassan (AS) would gather his children,his niece and nephews and tell them:
You may be considered as kids now, but it is so likely that you will be the elders of many groups and families in the future. Therefore, it is your duty to seek for knowledge. And if there are some people among you who cannot memorize their knowledge, they must write and put their writings in their houses and keep them safe. (11)

Carriers of Books

Action and knowledge are inseparable in the Islamic view. If you act without having enough knowledge and data about the work you’re going to do, you will spoil it and your efforts will be in vain. Imam Sadiq (AS) says:
The one who acts without having enough knowledge is like a passenger who goes on the wrong road. So they get farther and farther from their destination as they go faster and faster. (12)
On the other hand, knowledge without acting is also useless. Imam Ali (AS) says:
Knowledge without action is like a tree without fruit or like a bow without its string (13).
The Holy Quran also says:
The example of those who were charged with the Torah (Old Testament), then failed to carry it, is that of a donkey carrying books. Evil is the example of the people who deny Allah’s signs, and Allah does not guide the wrongdoing lot. (62:5)

Resources

1.  Fotooh al-Boldan, Pg.473
2.   Al-She’r va ash-Sho’ara, Pg.334
3.   As-Sahih fi sirah an-Nabi al-A’zam, Vol.1, Pg.49
4.   Vasa’il al-Shia, Vol.18, Pg.14, number 20 & 23
5.   Nahj al-Balaghah, Utterance no.147
6.   Kafi, vol.1, pg.3
7.   At-Tabaghat al-Kubra, vol.2, pg.22
8.   Bihar al-Anvar, vol.2, pg.151
9.   Kafi, vol.1, pg.52
10.   Kafi, vol.1, pg.52
11.   Kanz al-Ummal, vol.1, pg.752
12.   Tohaf al-Uqul, pg.362
13.   Mizan al-Hikmah, vol.4, pg.2841

As a kind of solution for decreasing the financial problems of the Islamic society, especially those of underprivileged people, Islam has offered some ways that one of the most effective of which is Khums. In the literal sense, Khums in Islam means one-fifth of something and in the sharia of Islam is one of the most important financial mandatory rules and is generally defined as paying one-fifth of the remainder of your yearly income.

Generally speaking, Khums becomes obligatory in seven cases, but the one which is inscribed to income is considered as the most salient kind. In this case, one has to pay one-fifth of what has remained from his income after subtracting his own expenses on the exact date that he has paid Khums in the previous year; in other words, one should specify a date on which he would pay his Khums every year.

To Which Incomes Khums in Islam Belongs?

There are certain kinds of income that would make the payment of Khums obligatory: agricultural income, commercial and trading income, income earned through renting something (e.g., house, car, etc.), the income that one earns through working for someone else.

On the other hand, in some cases, paying Khums is not necessary anymore, including the inherited money, gifts, rewards, marriage portion (Mahrieh)[i], mortmain property (Waqf), borrowed money, money paid by the insurance company, money paid as a scholarship to university students, money the Khums of which has been paid once, etc[ii] [2].

To whom Should We Pay Khums in Islam?

Basically, Khums is divided into two parts, one is given to Imam – who is your Religious expert (Marja Taqlid) or other qualified religious experts - and the other part is for descendants of Prophet Muhammad (PBUH&HP) (Sadaat) who are poor, orphan or faced with difficulty on their journey.

Why Do We Pay Khums in Islam?

Generally speaking, paying Khums brings about two kinds of effects, in two scales: first of all spiritual effects upon each and second, financial advantages upon the whole society. 

1. Spiritual and Individual:

As one of the forms of serving God, Khums should be paid with the intention of Allah’s satisfaction; accordingly firm belief and true faith guarantees performing this task. What’s more, engaging in this activity will arise a sense of generosity and philanthropy in the person, wipes away greediness and avarice from his soul and provides the necessary condition for benefiting from Allah’s spiritual and material blessings.

Furthermore, by paying a certain amount of money to the religious experts (Marja Taqlid), one feels involved in the practice of spreading the religion and Islamic ideology in the society and will always remain in the right side, helping the people who follow God’s commands against those who have transgressed from His way.

Another result of performing the holy task of Khums is that it will make the person more financially organized and dutiful, feeling responsible for underprivileged Muslims in the society and striving in the way of Allah and His Prophet (PBUH&HP).

2. Financial and Social:

As we already know, one of the main reasons for sending prophets was to provide the necessary grounds for establishing social justice through people themselves. So, they brought many rules that would pave their way to achieve this aim, one of the most important of which is paying Khums.

In fact, Khums acts as an equalizer of wealth within the Islamic community; Each individual with the intention of God’s satisfaction and true faith, donate a part of the remainder of his earnings to one of the most reliable, faithful and God-fearing people in the society - religious expert - in order for him to spend it for the purpose of improving the society. 

Paying Khums provides the sufficient resources for the people who are engaged in preaching Islam in the world and in some way reinforce the Islamic government. This money will help religious experts to spend their time and energy in inviting people to religion, answering their doubts and clarifying Islamic rules and regulations for them.

Moreover, this holy task will simultaneously produce two effects in Islamic society: involving religious experts in people’s financial difficulties will result in a closer and stronger relationship between them on one hand, and being responsible to give certain amount of money away, on the other hand, will make people more attentive to one another’s problems, create a bond between different classes within the society and reduce the gap between them through fairly distributing the wealth. 

Notes:

[i]. A mandatory payment, in the form of money or possessions paid by the groom, or by the groom's father, to the bride at the time of marriage that legally becomes her property [1].

[ii]. It is noteworthy that the cases above are varied according to different religious experts.

References:

1. Mahr
2. Ayatollah Khamenei, Resale Amuzeshi (didactic treatise), Khums rules and regulations