How should he call his friend to the way of al-Sunnah wa’l-Jamaa’ah?
Question

                How can I call a Muslim brother of mine who is a Sufi? He has been very kind to me, and he always attends lessons with the scholars, but I have no one among the scholars who could help me in that. You know how difficult it is to convince the Sufis, especially when the whole environment is Sufi and he accuses the Salafis of being quick to label people as kaafirs etc.
Praise be to Allah.We ask 
    Allaah to reward you with good for your keenness and your interest in 
    guiding your brother in matters of religion. Undoubtedly one of the greatest 
    blessings of Allaah to His slave is when He makes him active in da’wah, 
    calling others to Him and enjoining what is good and forbidding what is 
    evil. 
The daa’iyah 
    who calls people to Allaah needs knowledge and insight when he calls 
    people. 
Shaykh Ibn 
    ‘Uthaymeen (may Allaah have mercy on him) said: 
O youth who 
    seeks to call people to Allaah, ponder the verse in which Allaah says 
    (interpretation of the meaning): 
“Say (O 
    Muhammad): This is my way; I invite unto Allaah (i.e. to the Oneness of 
    Allaah — Islamic Monotheism) with sure knowledge, I and whosoever follows me 
    (also must invite others to Allaah, i.e. to the Oneness of Allaah — Islamic 
    Monotheism with sure knowledge). And Glorified and Exalted be Allaah (above 
    all that they associate as partners with Him). And I am not of the 
    Mushrikoon (polytheists, pagans, idolaters and disbelievers in the Oneness 
    of Allaah; those who worship others along with Allaah or set up rivals or 
    partners to Allaah)”
[Yoosuf 
    12:108]
i.e., with 
    insight into the message to which you are calling, and with insight into the 
    state of the one who is called, and with insight into the manner of calling, 
    for there are conditions to which you must pay attention, including the 
    following: 
1 – The 
    daa’iyah must have insight into the message to which he is calling people. 
    He should have knowledge of the Islamic ruling on that to which he is 
    calling others, because he may think that he is calling them to something 
    that he thinks is obligatory when in Islam it is not obligatory, so he is 
    obliging the slaves of Allaah to do something that Allaah does not oblige 
    them to do. Or he may call them to give up something that he thinks is 
    haraam, when in the religion of Allaah it is not haraam. So he is forbidding 
    to the slaves of Allaah something which Allaah has permitted to them.  
2 – He 
    should have insight into the state of the one whom he is calling. It is 
    essential to know the situation of the one whom he is calling: what is his 
    level of knowledge, what is his ability in arguing, so that you can prepare 
    yourself to debate with him. Because if you enter into a discussion with 
    such a one, and you lose the argument because of his skill in arguing this 
    will be a great disaster for the truth and you will be the cause of it. You 
    should never think that the one who argues in support of falsehood will fail 
    in his argument in every case. 
3 – He 
    should have insight into the manner of calling. I encourage my brother 
    daa’iyahs to use wisdom and deliberation, for they know that Allaah says 
    (interpretation of the meaning): 
“He 
    grants Hikmah [wisdom] to whom He wills, and he, to whom Hikmah is granted, 
    is indeed granted abundant good. But none remember (will receive admonition) 
    except men of understanding”

    [al-Baqarah 2:269] 
End quote. 
Fataawa 
    al-Haram al-Makki, p. 1063-1066. 
See also 
    question no. 2023. 
Here we 
    would like to point out several things: 
1 – Do not 
    generalize, such as saying that the original idea behind Sufism is 
    innovation and misguidance, or that all Sufis are misguided. Rather you 
    should be cautious in your speech, such as saying: Whoever does such and 
    such or says such and such, whether he is a Sufi or otherwise, has fallen 
    into bid’ah, etc. 
The scholars 
    have not ruled that all Sufis are misguided, rather they have divided Sufism 
    into various categories, and explained what is in accordance with the Sunnah 
    and what goes against it. 
Shaykh 
    Muhammad ibn Ibraaheem said: 
Sufis fall 
    into two categories: Sunni Sufis and innovator Sufis. The good ones among 
    them have only a little bid’ah, but some of them have a great deal of 
    bid’ah, and some understood Sufism as supporting the belief in wahdat 
    al-wujood (a form of pantheism).  End quote. 
Majmoo’ 
    Fataawa Ibn ‘Ibraaheem, 1, no. 192. 
But most 
    Sufis nowadays have fallen into innovation and misguidance. 
This has 
    already been discussed in the answers to questions no.
    4983,
    47431 and
    20375 
You can 
    start with him in this manner, then measure the kind of Sufism that your 
    friend follows against the Qur’aan and Sunnah. If it is true, then praise be 
    to Allaah, but if it is false he will leave it in sha Allaah.
 Secondly: 
With regard 
    to his accusation that Salafis are quick to label others as kaafirs, this is 
    an accusation that we have heard often. Regarding as a kaafir one for whom 
    there is proof that he is a kaafir is not wrong or shameful, rather it is 
    wrong not to regard as a kaafir one for whom there is proof that he is a 
    kaafir. We do not deny that some of those who claim to be Salafis hasten to 
    label others as kaafirs, but this is not the way of Ahl al-Sunnah 
    wa’l-Jamaa’ah. Ahl al-Sunnah (those who adhere to the Sunnah) do not label 
    anyone as a kaafir merely because of a sin that he commits, even if it is a 
    major sin. Rather it is essential to establish shar’i proof that this action 
    constitutes kufr, then certain conditions must be met and certain 
    impediments avoided in order for a person to be judged to be a kaafir. 
Shaykh 
    al-Islam Ibn Taymiyah (may Allaah have mercy on him) said: 
The view of 
    Ahl al-Sunnah wa’l-Jamaa’ah is that they do not regard any of the people of 
    the qiblah as kaafirs simply because of sins, or because of 
    misinterpretations, rather if an individual has both good deeds and bad 
    deeds, then his case is up to Allaah. End quote.
Majmoo’ 
    al-Fataawa, 27/478 
And he (may 
    Allaah have mercy on him) said:  
No one has 
    the right to regard any of the Muslims as a kaafir, even if they make 
    mistakes or do wrong things, until proof is established. If it is proven for 
    certain that a person is a Muslim, that cannot be altered by doubt alone, 
    rather it can only be altered after establishing proof and dispelling doubt. 
    End quote. 
Majmoo’ 
    al-Fataawa, 12/466 
And he 
    explained that Ahl al-Sunnah do not describe as kaafirs those who differ 
    from them, even if the one who differs from them may hasten to label Ahl 
    al-Sunnah as kaafirs. 
He said: 
The imams of 
    Ahl al-Sunnah wa’l-Jamaa’ah and the people of knowledge and faith have the 
    qualities of knowledge, justice and mercy. They know the path of truth and 
    they know that if they follow this path they will be in accordance with the 
    Sunnah and free from innovation, and they treat justly those who differ from 
    that, even if they wrong them, as Allaah says (interpretation of the 
    meaning): 
“Stand 
    out firmly for Allaah as just witnesses; and let not the enmity and hatred 
    of others make you avoid justice. Be just: that is nearer to piety”

    [al-Maa’idah 5:8]
They are 
    merciful towards people and they desire good, guidance and knowledge for 
    them, and they do not wish them ill. Hence the people of knowledge and 
    Sunnah do not regard as kaafirs those who differ from them, even if the one 
    who differs from them regards them as kaafirs, because what constitutes kufr 
    is the matter of shar’i rulings. End quote. 
Al-Radd 
    ‘ala al-Bakri, p. 256-258 
Thirdly: 
If you do 
    not have enough knowledge to convince him and answer the doubts that he has, 
    then you can convey the truth to him by means of useful books and tapes, or 
    by visiting scholars in your own city or elsewhere, to listen to them and 
    ask them questions. The means of good are still available to all people 
    these days – praise be to Allaah – so do not scorn such things. Perhaps a 
    tape that you tell him about may be the means of his being guided. 
Fourthly: 
Do not 
    despair of calling him and guiding him, even if it takes a long time. How 
    many people have repented and come back to the truth after years of being 
    called and reminded. 
And Allaah 
    knows best.