Bad attitude on the part of the daa‘iyah is an obstacle to da‘wah
Question

The Messenger of Allah (blessings and peace of Allah be upon him) said: “There is nothing that will weigh more heavily in the balance of the believer on the Day of Resurrection than a good attitude, and verily Allah hates the foulmouthed person who speaks in an offensive manner.” And he (blessings and peace of Allah be upon him) said: “Shall I not tell you of the dearest of you to Allah, and the ones who will be seated closest to me on the Day of Resurrection?” They said: Yes, O Messenger of Allah. He said: “The best of you in attitude.”
Does a bad attitude invalidate faith or cancel it out? Does a bad attitude nullify good deeds? Or is bad attitude a sign of weakness of faith?
What if a person who is a practising Muslim persists in having a bad attitude after being advised and warned? Is such a person regarded as persisting in sin? What is the harmful impact of a bad attitude on da‘wah or calling people to Allah, may He be glorified and exalted?
Finally, what do we say to practising Muslims who persist in their shameful bad attitude and show bad manners towards Muslims who are sinners, and non-Muslims?
Please advise us about these issues, may Allah bless you.
Praise be to Allah.Firstly: 
At-Tirmidhi (2002) narrated – and classed the report as saheeh – from Abu’d-Darda’ that the Prophet (blessings and peace of Allah be upon him) said: “There is nothing that will weigh more heavily in the balance of the believer on the Day of Resurrection than a good attitude, and verily Allah hates the foulmouthed person who speaks in an offensive manner.” Classed as saheeh by al-Albaani in Saheeh at-Tirmidhi. 
At-Teebi (may Allah have mercy on him) said: 
The phrase, “and verily Allah hates the foulmouthed person who speaks in an offensive manner” balances the phrase “the heaviest thing that may be placed in the balance”, so as to indicate that the lightest thing that may be placed in the balance is a bad attitude, and that a good attitude is the dearest of things to Allah, and a bad attitude is the most hateful of things, because offensive and foulmouthed talk are the worst thing in the scale of bad manners. 
End quote from Mirqaat al-Mafaateeh (8/3177) 
Ahmad (6735) narrated from ‘Amr ibn Shu‘ayb, from his father, from his grandfather, that he heard the Prophet (blessings and peace of Allah be upon him) say: “Shall I not tell you of the dearest of you to me and the ones who will be seated closest to me on the Day of Resurrection?” The people remained silent, and he repeated it two or three times, then the people said: Yes, O Messenger of Allah. He said: “The best of you in attitude.” 
Classed as saheeh by al-Albaani in Saheeh at-Targheeb (2650) 
At-Tirmidhi (2018) narrated – and classed the report as hasan – from Jaabir that the Messenger of Allah (blessings and peace of Allah be upon him) said: “Among the dearest of you to me and those who will be seated closest to me on the Day of Resurrection are the best of you in attitude, and the most hateful of you to me and those who will be seated furthest away from me on the Day of Resurrection are those who talk too much, those who talk down to people in an affected manner, and those who are long winded and pompous.”
Classed as saheeh by al-Albaani in Saheeh at-Tirmidhi. 
Al-Bukhaari (6035) and Muslim (2321) narrated from ‘Abdullah ibn ‘Umar that the Messenger of Allah (blessings and peace of Allah be upon him) said: “Verily the best of you are the best of you in attitude.” 
An-Nawawi (may Allah have mercy on him) said: 
This hadith offers encouragement to develop a good attitude and highlights the virtue of the one who has it, for it is a trait of the Prophets and close friends of Allah, may He be exalted. Al-Hasan al-Basri said: The definition of a good attitude is to do acts of kindness, refrain from harming others and to show a cheerful countenance. Al-Qaadi ‘Iyaad  said:  It means mixing with people in a gentle and cheerful manner, being friendly and compassionate towards them, being forbearing towards them and overlooking their mistakes, being patient with them at times of difficulty, not being arrogant or putting oneself above them, and avoiding harshness, anger and censure. End quote. 
Secondly: 
A good attitude increases faith, whereas a bad attitude decreases faith, but does not invalidate it altogether, because faith increases with acts of obedience and decreases with acts of disobedience. 
Part of perfect faith is a good attitude; the Muslim who has a bad attitude is lacking in faith. 
Abu Dawood (4682) narrated that Abu Hurayrah said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “The most perfect of the believers in faith are the best of them in attitude.”
Classed as saheeh by al-Albaani in Saheeh Abi Dawood
Ahmad (20831) narrated that Jaabir ibn Samurah said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Obscene words and deeds are not part of Islam; the best of the people in Islam are the best of them in attitude.”
Classed as hasan by al-Albaani in Saheeh at-Targheeb (2653) 
This is what indicates that a bad attitude detracts from obligatory faith.
Please see the answer to question no. 10809 for more information on things that detract from faith. 
Thirdly: 
The basic principle is that righteous deeds are not rendered invalid by anything except the ascription of partners to Allah, may He be exalted. Allah, may He be exalted, says (interpretation of the meaning): “And whosoever disbelieves in the Oneness of Allah and in all the other Articles of Faith (i.e. His (Allah’s), Angels, His Holy Books, His Messengers, the Day of Resurrection and AlQadar (Divine Preordainments)), then fruitless is his work, and in the Hereafter he will be among the losers” [al-Maa’idah 5:5]. 
One of the main differences between Ahl as -Sunnah and the groups who follow innovation, such as the Khaarijis, Mu‘tazilis and others of that ilk, is that they believe that acts of obedience may be rendered invalid by sin. 
Ahl as-Sunnah, on the other hand, say that major sin does not render righteous deeds invalid, although faith is decreased by sins just as it is increased by acts of obedience. 
However, it is narrated in the texts that there are some sins for which Allah warns of invalidation of good deeds for the one who commits them, but that does not apply to every sin; rather it applies to some specific sins, concerning which this specific warning was narrated. This ruling should be limited to that, and no analogy can be drawn applying it to other sins, as Allah, may He be exalted, says (interpretation of the meaning): “O you who believe! Raise not your voices above the voice of the Prophet (blessings and peace of Allah be upon him), nor speak aloud to him in talk as you speak aloud to one another, lest your deeds may be rendered fruitless while you perceive not” [al-Hujuraat 49:2]. 
In the answer to question no. 81874, we explained that sins and innovations invalidate the reward for their counterparts among good deeds, by way of punishment. 
Please see also the answer to question no. 107241
Fourthly: 
The practising Muslim who adheres to Islamic etiquette cannot have a bad attitude; rather a bad attitude only results from going against the teachings of Islam and keeping company with bad people. Worse than that is the failure to try to correct the bad attitude that one has and not accepting the advice of anyone who advises him and calls him to do that. Allah, may He be exalted, says (interpretation of the meaning):
“And march forth in the way (which leads to) forgiveness from your Lord, and for Paradise as wide as are the heavens and the earth, prepared for Al-Muttaqoon (the pious).
Those who spend (in Allah’s Cause - deeds of charity, alms, etc.) in prosperity and in adversity, who repress anger, and who pardon men; verily, Allah loves Al-Muhsinoon (the good-doers).
And those who, when they have committed Faahishah (illegal sexual intercourse etc.) or wronged themselves with evil, remember Allah and ask forgiveness for their sins; - and none can forgive sins but Allah - And do not persist in what (wrong) they have done, while they know”
[Aal ‘Imraan 3:133-135]. 
Fifthly: 
A bad attitude is very harmful to efforts to call people to Allah (da‘wah), because people will be put off by the one who has a bad attitude and treats people badly; they will reject his advice and will say, “Let him advise himself first!” But people will like the one who has a good attitude and treats them kindly; they will accept his advice and listen to what he says. 
Whoever has a bad attitude will adversely affect da‘wah efforts and will make people think badly of practising Muslims, and will make them think that they do not treat people well, and that will put them off. 
A bad attitude will also lead to bad consequences when dealing with non-Muslims, because a bad attitude will put them off Islam and Muslims. 
So let people fear their Lord, lest their attitude and deeds be a cause of turning people away from the path of Allah or being put off from Islam and Muslims. 
It was narrated that Abu Mas‘ood al-Ansaari said: A man came to the Messenger of Allah (blessings and peace of Allah be upon him) and said: I keep away from Fajr prayer because of So and so, because he makes it too long for us. I have never see the Prophet (blessings and peace of Allah be upon him) so angry in exhortation as he was that day. He said: “O people, there are among you some who put others off. Whoever among you leads the people in prayer, let him be brief, for among them are the elderly, the weak and those with urgent needs.” 
Narrated by al-Bukhaari (702) and Muslim (466). 
Al-Mannaawi (may Allah have mercy on him) said: 
A bad attitude will lead to trouble and evil consequences for the individual and others, because it leads the individual to shame in this world and, in the hereafter, to the Fire. The poet said: 
How many young men have been hindered by a bad attitude, so they ended up blameworthy, with little to be praised for. 
And they said that whoever develops a bad attitude should be forsaken. 
And they said that a bad attitude is indicative of an evil nature and meanness of character. 
And they said that people with bad attitudes could almost be regarded as being among the animals. 
Whoever is blessed with a good attitude, congratulations to him, otherwise he should deal with his bad attitude until it is gone. 
End quote from at-Tayseer (2/121). 
However, we would like to point out the following: 
Many people are unfair towards religious people and they do not view them in a just manner; when they see a small fault in them, they regard it as abhorrent and evil. Rather, even if a religious person does something that is permissible, or seeks some worldly matter that is allowed, they attack him viciously and start gossiping about him. 
If he makes a minor mistake, or falls into something that no human is completely safe from, they go all out and raise a hue and cry. Some people want religious people to be like the Prophets themselves! 
Some of them watch out for the religious person to make a mistake and slip up, so that they can launch a heavy attack on him, and on the religion and the people who adhere to it. Allah, may He be exalted, loves justice and fairness, and enjoins everyone to treat everyone else fairly, no matter who he is. 
Allah enjoins His slaves to regard people and treat them as they would like others to regard and treat them. 
Muslim narrated in his Saheeh (1844) that ‘Amr ibn al-‘Aas (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “There was no Prophet before me but it was his duty to tell his ummah of the best of what he knew was good for them and warn them about the worst of what he knew was bad for them. The time of peace and security for this ummah has been made in its first era, and its last era will be afflicted with trials and things that you will be confused about. Fitnah (tribulation) will come in waves, one after another. A fitnah will come and the believer will say, ‘This is going to cause my doom.’ Then when it ends, another fitnah will come, and the believer will say, ‘This is the one.’ Whoever would like to be delivered from Hell and enter Paradise, let him die believing in Allaah and the Last Day, and let him treat people as he would like to be treated 
And Allah knows best.