Relationship of Sin and Attitude to Aqeedah
Question

Does falling into sin indicate that there is something wrong with one’s ‘aqeedah (belief system)?
Praise be to Allah.Good attitudes – which are those that are obedience in and of themselves or lead to obedience – are part of Islam, in fact they are Islam. Allaah praised His Prophet Muhammad (peace and blessings of Allaah be upon him) for being “on an exalted standard of character” and Ibn ‘Abbas interpreted khuluq (translated here as “character”) as meaning Islam. 
Allaah says (interpretation of the meaning): 
“And verily, you (O Muhammad) are on an exalted (standard of) character”
[al-Qalam 68:4]
Ibn ‘Abbaas (may Allaah be pleased with him) said: You are on an exalted standard of religion, which is Islam. Narrated by al-Tabaraani in his Tafseer (12/179). 
The correct view is that character cannot be separated from religion. Al-Fayroozabaadi said in his book Basaa’ir Dhawi’l-Tamyeez (2/568): It should be noted that religion is all character. Whoever excels over you in character will excel over you in religious commitment. End quote. 
Undoubtedly ‘aqeedah (belief) has a strong connection to one’s conduct and character, negative or positive. That is clear from a number of things, including the following: 
1 – The Muslim who believes that Allaah can hear him and see him and knows his secrets, and that belief is very strong in him, will be affected by this and will not do anything that a person whose belief in these matters is weak might do. 
Among the evidence for this is the following: 
(a)The verse in which Allaah says (interpretation of the meaning):
“But if you do good and keep away from evil, verily, Allaah is Ever Well‑Acquainted with what you do”
[al-Nisa’ 4:128]
(b)The verse in which Allaah says (interpretation of the meaning):
“O you who believe! Stand out firmly for justice, as witnesses to Allaah, even though it be against yourselves, or your parents, or your kin, be he rich or poor, Allaah is a Better Protector to both (than you). So follow not the lusts (of your hearts), lest you avoid justice; and if you distort your witness or refuse to give it, verily, Allaah is Ever Well‑Acquainted with what you do”
[al-Nisa’ 4:135]
(c)The verse in which Allaah says (interpretation of the meaning):
“Verily, Allaah commands that you should render back the trusts to those, to whom they are due; and that when you judge between men, you judge with justice. Verily, how excellent is the teaching which He (Allaah) gives you! Truly, Allaah is Ever All‑Hearer, All‑Seer”
[al-Nisa’ 4:58]
2 – The Muslim who believes in the promises and warnings of Allaah will be motivated by that belief to do that which is beloved to Allaah, and to keep away from everything that is hated by Him. 
It was narrated that Abu Hurayrah said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The most perfect of the believers in faith is the one who is best in attitude.” Narrated by al-Tirmidhi (1162) and he said: it is hasan saheeh. Also narrated by Abu Dawood (4682). 
Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said: 
It is well known that the most beloved of His creation to Him are the believers, and if the most perfect of them in faith are those who are best in attitude, then the most beloved to him are those who are best in attitude, and khuluq (character, attitude) is religion as Allaah says (interpretation of the meaning): “And verily, you (O Muhammad) are on an exalted (standard of) character” [al-Qalam 68:4]. Ibn ‘Abbaas said: On a high standard of religion. It was also interpreted thus by Sufyaan ibn ‘Uyaynah, Ahmad ibn Hanbal and others, as we have explained elsewhere. Al-Istiqaamah (p. 442). 
Al-Mubaarakfoori (may Allaah have mercy on him) said: He said, “The most perfect of the believers in faith is the one who is best in attitude” because perfection of faith implies a good character and good treatment towards all people. 
Tuhfat al-Ahwadhi (4/273). 
3 – Strength of faith motivates one to do righteous deeds, and prevents one from indulging in sin. 
This is indicated by the following: 
(a)It was narrated from Abu Hurayrah that the  Prophet (peace and blessings of Allaah be upon him) said: “No adulterer is a believer at the time when he is committing adultery; no thief is a believer at the time when he is stealing; no drinker of wine is a believer at the time when he is drinking it.” Narrated by al-Bukhaari (2334) and Muslim (57).
(b)It was narrated from Abu Shurayh that the Prophet (peace and blessings of Allaah be upon him) said: “By Allaah he does not believe, by Allaah he does not believe, by Allaah he does not believe.” It was said: Who, O Messenger of Allaah? He said: “The one from whose harm his neighbour is not safe.” Narrated by al-Bukhaari (5670).
(c)It was narrated from ‘Abd-Allaah ibn ‘Umar that the Messenger of Allaah (peace and blessings of Allaah be upon him) passed by an Ansaari man who was exhorting his brother to be modest. The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Let him be, for modesty is part of faith.” Narrated by al-Bukhaari (24) and Muslim (36).
Maalik ibn Dinar (may Allaah have mercy on him) said: Faith starts in the heart weak and feeble like a plant. If its owner takes care of it and nourishes it with beneficial knowledge and righteous deeds, and keeps away from it weeds and things that will make it weak, then soon it will grow and increase and will develop roots and branches, and will bear fruit and provide shade endlessly, until it becomes like a mountain. But if its owner neglects it and does not take care of it, a goat will come and eat it, or a child will come and take it, and the weeds will grow and overshadow it and destroy it. The same applies to faith. 
Khaythamah ibn ‘Abd al-Rahmaan said: Faith grows strong in fertile soil and grows weak in arid soil. Its fertile soil is righteous deeds and its arid soil is sin and disobedience. Quoted by Ibn Taymiyah in al-Eemaan, p. 213. 
4 – By the will and decree of  Allaah, faith prevents many bad attitudes and sins against which Islam issues stern warnings, such as getting angry, rending one’s garment, tearing out one’s hair and wailing. Faith also calls a person to acquire the best of attitudes, such as patience, acceptance and seeking reward. It was narrated that Suhayb al-Roomi (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “How wonderful is the situation of the believer, for all his affairs are good. If something good happens to him, he gives thanks for it and that is good for him; if something bad happens to him, he bears it with patience, and that is good for him. This does not apply to anyone but the believer.” Narrated by Muslim (2999). 
In Sunan Abi Dawood (4700) it says: ‘Ubaadah ibn al-Saamit said to his son: You will never taste the reality of faith until you understand that whatever befalls you would never have missed you, and whatever misses you would never have befallen you. I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: “The first thing that Allaah created was the Pen, and he said to it: ‘Write.’ 
It said: ‘O Lord, what should I write?’ 
He said: ‘Write the decrees of all things until the Hour begins.’” 
O my son, I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: “Whoever dies believing in something other than this does not belong to me.” Classed as saheeh (authentic) by al-Albaani. 
5 – Islam urges us to do a great deal of good deeds, confirming their connection to belief in Allaah and the Last Day, and it forbids sins and deeds that incur punishment by reminding us of belief in Allaah and the Last Day. 
This is indicated by the following: 
(a)It was narrated that Abu Hurayrah said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Whoever believes in Allaah and the Last Day, let him honour his neighbour; whoever believes in Allaah and the Last Day, let him honour his guest; whoever believes in Allaah and the Last Day, let him speak good or else remain silent.” Narrated by al-Bukhaari (5672) and Muslim (47).
(b)It was narrated from ‘Abd-Allah ibn ‘Umar that the Prophet (peace and blessings of Allaah be upon him) said: “It is not permissible for a woman who believes in Allaah and the Last Day to travel for a distance of three nights, unless she has a mahram (close male relative whom she can never marry) with her.” Narrated by al-Bukhaari (1036)and Muslim (1338).
(c)It was narrated that Umm Habeebah said: I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: “It is not permissible for a women who believes in Allaah and the Last Day to mourn for more than three days for anyone who dies, except for a husband, four months and ten days.” Narrated by al-Bukhaari (1221) and Muslim (1486).
6 –The Prophet (peace and blessings of Allaah be upon him) explained in his Sunnah (prophetic teachings) that false belief, such as hypocrisy, leads to bad attitudes and bad deeds. 
It was narrated from Abu Hurayrah that the Prophet (peace and blessings of Allaah be upon him) said: “The signs of the hypocrite are three: when he speaks he lies, when he makes a promise he breaks it, and when he is entrusted with something he betrays that trust.” Narrated  by al-Bukhaari (33) and Muslim (59). 
Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said: Those who go against the people of hadeeth (ahl al-sunnah-followers of the Prophet) are those who are likely to have bad deeds, either because of corrupt beliefs and hypocrisy, or because of a sickness in the heart and weakness of faith. Among them are those who neglect obligatory duties, transgress the limits, take rights and duties lightly and are hard hearted, as is clear to everyone. Most of their Shaykhs are accused of major sins even if there are among them some who are known for asceticism and worship. The asceticism and worship of some of the common folk of Ahl al-Sunnah are better than what they do. 
It is well known that knowledge is the basis of action, and sound roots produce sound branches. A man does not do evil actions except for two reasons, either need or ignorance. The one who is aware of the abhorrence of a thing that he has no need of will not do it, unless his whims and desires have overpowered his reason and led him to commit sin, which is another matter altogether. Majmoo’ al-Fataawa (4/53). 
We ask Allaah to set all our affairs straight and to guide us to the best of words, deeds and attitudes. 
And Allaah knows best.