1. Causes of Decrease in Faith
Disobediance and
Sinful Deeds

2. Faith decreases with disobediance however Allah tells us that sins vary greatly with respect to their rank, the evils they entail and the intensity of their harm.
"They are in varying grades before Allaah and Allaah is All-Seer of what they do." [Soorah Aal-'Imraan (3):163]
"For all there will be degrees, according to that which they did. And your Lord is not unaware of what they do." [Soorah al- An'aam (6):132]
"The postponing (of a sacred month) is indeed an addition to kufr ..." [Soorah at- Tawbah (9):37]
"... As for those who believe, it has increased their eemaan and they rejoice. But as for those whose hearts is a disease, it will add rijs [i.e., suspicion, doubt and disbelief. [t]] to their rijs." [Soorah at- Tawbah (9): ]

3. Both the Quran and sunnah show that sins cane be divided into major and minor categories
Allah says in the interpretation of the meaning:
"And if you avoid the major of the sins that you are forbidden from, We shall remit for you your (minor) sins and admit you to a noble entrance." [Soorah an-Nisaa' (4):31]
"Those who avoid major sins and al-Fawaahish except the small faults ..." [Soorah an-Najm (53):32]
It is recorded in Saheeh Muslim, that Aboo Hurayrah relates the Messenger of Allaah as saying: "The five prayers, the Friday prayer to the (next) Friday prayer and Ramadaan to (the next) Ramadaan; they expiate all (sins committed) between them as long as the major ones are avoided."

4. Also from the sunnah:
"We were with the Messenger of Allaah when he questioned three times, "Shall I not inform you o f the greatest of the major sins? Committing shirk with Allaah, disobedience to parents and issuing a false testimony" – [Bukhari]
The Messenger of Allaah was asked, ‘Which sin is the greatest before Allaah?’ He replied, ‘That you ascribe to Allaah an equal whilst it is He who created you.’ It was asked, ‘Then which?’ He said, ‘That you kill your child in fear that he will also need nourishment alongside you.' It was asked, 'Then which?' He answered, ‘That you have a sexual relationship with the wife of your neighbor’ - [Bukhari]

5. Malikiyyah Shaytaaniyyah Sab'iyyah Baheemiyyah
Many other texts also show the varying levels of sins and that they are classified into major and minor ones. Furthermore, from another perspective, these sins can be classified into four categories:
Malikiyyah
Shaytaaniyyah
Sab'iyyah
Baheemiyyah

6. Malikiyyah Sins
Are sins committed when one takes upon himself qualities of Lordship, which are not befitting for him, such as majesty, pride, domination, subjugation, highness, enslavement of people and other qualities of this type. Sins belonging to this category are of the most serious type.

7. Shataniyyah Sins
These are sins resembling Shaytaan in envy, oppression, cheating, hatred, deception, plotting evil, ordering disobedience of Allaah and beautifying it, prohibiting obedience of Allaah and censuring it, committing innovations in the religion of Allaah and the call to innovations and deviation. This category is placed second to the first type with respect to its harms, even though its harms are lesser.

8. Sabiyyah Sins
These are are sins of transgression, anger, spilling blood, pouncing upon the weak and incapable. This gives birth to many types of harms to fellow humans and boldness towards committing oppression and transgression.

9. Baheemiyyah Sins
These are sins such as gluttony and the desire to satisfy lusts of the stomach and genital organs. This gives rise to fornication, adultery, theft, consumption of wealth belonging to orphans, miserliness, avarice, cowardice, restlessness, impatience and much more.

10. Baheemiyyah sins Are the stepping stones to the other category of sins
Most people fall into these sins due to their inability to commit sins in the other categories.
But eventually they end up moving up to sins in the other categories

11. The degree of its harmful effects,
This diversity of sins and of their effect upon eemaan trace back to a variety of considerations. Some of these are:
The class of the sin,
Its amount,
The degree of its harmful effects,
The place, the time, with regard to its doer

12. For Example:
The one who does the crime in secret is less sinful than the one who does it publicly.
The one who conceals his sin is less sinful than the one who relates it publicly to the people
having a sexual encounter with a woman who has no husband is less sinful than having a sexual encounter with one who has a husband, because of the oppression and transgression it entails against the husband as well as ruining his bed.
To have a sexual encounter with the wife of a neighbor is more severe than having a sexual encounter with one who is far from home, because of its connection to harming the neighbor and not maintaining the instruction of Allaah and His Messenger with regard to the neighbor.

13. Other Examples:
To fornicate or commit adultery in the days and nights of Ramadaan is more sinful than at any other time. Its occurrence in the blessed and virtuous places is more sinful than in any other places.
A sin committed by a scholar is worst than if the same were committed by someone ignorant, because of the former's awareness of its abhorrence and consequences.

14. The degree of its harmful effects The amount The place and time
In summary; sins decrease eemaan and the amount of decrease varies in accordance to a multitude of considerations. Some of these are:
The Class of the Sin
The degree of its harmful effects
The amount
The place and time
The indifference towards it
The Doer