Firm conviction and disobedience

Even though the belief in Allah and the fear of His retribution is present in man, he is still prone to sins. It is not that one who believes in Allah cannot sin. For example, who does not know that a dead body cannot cause any harm? Everyone firmly believes in it. Yet how many people can stay alone with a corpse at night? Here a firm belief is unable to rid the man of his fear of the dead. Even a firm belief cannot guarantee one's actions. It is for this reason that we recite in Dua, "Bestow upon me such a firm conviction that I can worship you with sincerity." There is no doubt that if one prays to Allah out of intense fear; which would restrains him from committing sins, Allah will surely create such a fear in his heart.

Also the magnitude of his fear increases along with the increase in grades of piety and obedience which he achieves due to constant efforts.

It has been mentioned in the traditions:
"Whoever desires something and strives for it, finds it."

Truly! Being absolutely truthful seems possible only for the Masoomeen (a.s.). As the Holy Quran states,

"And be with the Truthful Ones!"
(Surah Tauba 9:119)

The "Truthful Ones" referred to in this ayat are the Ahul Bayt (a.s.). 

According to the traditions 'false oath' belongs to the category of greater sins. False swearing is a greater sin as mentioned in the authentic tradition recorded by Abdul Azeem and also in the tradition of Imam Reza (a.s.) as quoted by Fazl Ibne Shazaan. There is a report from Imam Ja'far as-Sadiq (a.s.) in the book Tuhafful Uqool: 
"False swearing causes destruction of the faith." 
(Behaarul Anwaar)

According to traditions a person who conveys a false report or oath will be sunk in depravity and will be surrounded by hell fire. The traditions state that false oath destroys a person's faith just as a sharp blade removes hair from the body. To swear by Allah, to convey a falsehood is a particularly detestable sin.

Punishment for a false oath

"(As for) those who take a slender price for the covenant of Allah and their own oaths-surely-they shall have no portion in the hereafter, and Allah will not speak to them, nor will He look upon them on the Day of Resurrection nor will He purify them, and they shall have a painful chastisement." 
(Surah Aale Imraan 3:77)

This same verse was quoted by Imam Ja'far as-Sadiq (a.s.) to prove that taking a false oath is one of the greater sins.

The Prophet (s.a.w.s.) orders the taking of oath

An interesting incident is quoted in Tafseer al-Mizan from the book Amali. Imrul Qays and another man had a dispute regarding some property. Both of them came to the Holy Prophet (s.a.w.s.). He (s.a.w.s.) asked Imrul Qays,
"Can you provide two just witnesses to substantiate your claim?"

He replied, "No!"
The Holy Prophet (s.a.w.s.) said,
"Then your opponent should take an oath."

Imrul Qays said, "But what if he swears falsely and acquires my property?"

The Holy Prophet (s.a.w.s.) replied,
"If he swears falsely he shall be included among people who will not be eligible for Divine Mercy on the Day of Judgment and Allah shall not purify him of sins. There would be a dreadful punishment for such a man!"

When the litigant heard these statements he was filled with horror and gave up his false claim to the property of Imrul Qays.

Hazrat Imam Ja'far as-Sadiq (a.s.) is reported to have said:
"One who knowingly takes a false oath had made war upon Allah." 
(Al Kafi)

Imam Muhammad al-Baqir (a.s.) says that the Holy Prophet (s.a.w.s.) said:
"Refrain from false oath, because it destroys inhabitations and makes the sinner helpless." 
(Al Kafi)

According to other traditions, false oath and severing relations are two such sins that cause the destruction of towns and cities. The inhabitants are eliminated and the progeny terminated.

Evil consequences of false oaths

Hazrat Imam Ja'far as-Sadiq (a.s.) says,
"One who swears falsely becomes poor within forty nights" (means forty days). 
(Al Kafi)

Many traditions of similar connotations are available. The same Imam (a.s.) has also said:
"The false oath that takes one to the fire is the one which is taken to usurp the right of a Muslim or to usurp his property."

And Imam Ja'far as-Sadiq (a.s.) also remarks,
"When a person says, "Allah knows!" when in fact he is speaking a lie; then Allah tells him, 'Didn't you find anyone other than Me to ascribe the falsehood to?'
(Al Kafi)

The Holy Imam (a.s.) also says: 
"When a person says 'Allah knows' and the fact is that Allah knows contrary (to what he alludes), the heavens shudder due to the Might and Divine anger."

Types of oaths

An oath is taken to prove some fact or report, or to relate the same with emphasis. There are four kinds of oaths:
1. Wajib (Obligatory). 
2. Mustahab (Recommended).
3. Makrooh (Detestable).
4. Haraam (Prohibited).

When does it become wajib (obligatory) to swear?

It is wajib to take an oath in a situation where one's life or honour, or that of another Muslim, is in danger, and taking the oath can ward off the danger. When it is wajib to protect ones property it is also wajib to take an oath for its protection. In fact in all the above situations it is wajib even to take a false oath, although as a precautionary measure one should first try ones best to employ Toriya.