Question: Is it possible for a person to know whether they have iman? What is the sign of iman?
ANSWER
Iman [faith, belief] is to believe in the six pillars of belief expressed in the Amantu. It is declared in a hadith-i sharif:
(Iman is to have faith in Allah, in His angels, in His books, in His prophets, in the Last Day [that is, to believe in Doomsday, Paradise, Hell, Judgment, and Mizan] in qadar and that good and evil are from Allah, and in death and in Resurrection. It is to bear witness that there is no ilah except Allah and that I am a human slave and Messenger of His.) [Bukhari, Muslim, Nasai]
(It is necessary to confirm them in the heart as well.)

There are many qualities signifying that one has a strong faith. Some hadith-i sharifs on this matter are as follows:

(The Believer with the strongest faith is he who has good morals. Everybody can approach him easily, and he has many visitors. He gets on well with everyone. He who does not get on well with people around him is not a good one.) [Tabarani]

(He who does not forget Allahu ta’ala no matter where he is has a strong faith.) [Bayhaqi]

(He who loves Allah and His Messenger more than he loves all else, who loves another for the sake of Allah alone and who fears reverting to disbelief much more than being thrown into the Fire relishes the flavor of faith.) [Bukhari]

(You will not truly believe until you love one another.) [Tabarani]

(Haya is from iman.) [Bukhari]

(Cleanliness is half of iman.) [Muslim]

(Fulfilling one’s promises is from iman.) [Hakim]

(Being just, even if the outcome results in one’s own detriment, is from iman.) [Bazzar]

(Showing patience in time of disasters is from iman.) [Bazzar]

(Iman is naked. Piety is its dress; bashfulness is its adornment; fiqh is its capital; deeds are its fruits.) [Daylami]

(Iman has seventy-odd branches, the highest of which is to declare “La ilaha ill-Allah,” the lowest of which is to remove something harmful from a road. Haya, too, is a branch from iman.) [Tirmidhi]

(Iman means salat. He who performs salat minutely and by observing its due times and sunnats [and other precepts] is a Mu’min.) [I. Najjar]

(These three things are from iman: to spend in charity when you are in straits, to greet any Muslim you come across, and to act justly even if the outcome results in your own detriment.) [Nasai]

(A person possessing these qualities has a strong iman: He is not, in the slightest degree, afraid of becoming known in deeds that he does for the sake of Allah; he does deeds free from ostentation; and between two deeds, if one of them were beneficial to the next world and the other were beneficial to this world, he would prefer the one beneficial to the next world.) [Daylami]

(He who speaks ill of, who curses, who says vulgar words and who is shameless is not a perfect Mu’min.) [Bukhari]

(None of you has believed until I am more beloved to him than his parents, his children, and all humanity.) [Bukhari]

(He who does not wish for others what he wishes for himself cannot attain iman.) [Abu Ya’la]

When we have a look at the aforementioned hadith-i sharifs in a literal sense, they may give rise to such a misunderstanding that every clean person is a Believer. However, it is possible that a disbeliever may be clean, bashful, just, or abstain from adornment. To avoid this confusion, it is necessary to read the hadith-i sharifs in light of the explanations given by the scholars of hadith. If we read them without explanations, then we may misunderstand them.

So is the case with committing sins. It is not permissible to call a sinful Muslim a disbeliever. What is meant by the hadith “He who carries words among people will not enter Paradise” is that such a person cannot enter Paradise before bearing the penalty of his sins or without receiving forgiveness or intercession (shafa'at). In the same way, what is meant by the hadith “He who backbites deserves Hell” is that if one’s thawab weighs less than one’s sins, in such a case, backbiting leads a person to Hell.

The meaning of the hadith “A person who has an iota of conceit in his heart will not enter Paradise” is that the relevant person cannot enter Paradise without suffering its punishment first or without receiving forgiveness or intercession (shafa’at).

Furthermore, in a hadith-i sharif it is declared that a Mu’min can commit any sin with the exception of three sins, one of which is telling a lie. Outwardly, it can be inferred from this hadith-i sharif that one who tells a lie is not a Mu’min. However, it means such a person is not a perfect Mu’min. Also, it is stated that mendacity is one of the characteristics of hypocrisy. One who lies is not a hypocrite but has displayed one of the characteristics of hypocrisy.

Once, in response to such a question, “O the Prophet of Allah! Will a child who does not obtain the pleasure of his parents go to Hell even if they behave unjustly towards him?” our Prophet declared three times, “Yes, he who does not obtain the pleasure of them will go to Hell, even if they behave unjustly towards him.” From this hadith-i sharif, it may be concluded that those who do not attain the pleasure of their parents become disbelievers. But the situation is not so. It means they cannot go to Paradise without first suffering the punishment proportioned.

We should read such hadith-i sharifs as “A stingy person cannot enter Paradise” and “Stinginess is an act of kufr” with their explanations. Or else, they cause misapprehensions. Although stinginess is one of the vices, it is not an act of kufr [disbelief] after all. In fact, it means that a stingy person cannot enter Paradise without being subjected to the penalty of this sin. However, if one’s thawab weighs more than one’s sins, one enters Paradise without entering Hell at all. It is also possible that one can go to Paradise through receiving forgiveness and intercession.

The sign of true iman
Hadrat Imam-i Rabbani declares:
The symbol of true iman’s existence in the heart is to bear hostility against disbelievers and to annihilate the things that are peculiar to them and that are the symptoms of disbelief because Islam and kufr [disbelief] are opposites and antonyms of each other. To esteem one of them means to insult and to blame the other. To love the enemies of Allahu ta’ala and to cooperate with them draws one towards being hostile against Allahu ta’ala and His Prophet. A person thinks of himself as a Muslim, expresses the word tawhid, and says, “I believe,” and performs salat and every kind of worship, but on the other hand, cooperates with disbelievers. Yet one does not know that these loathsome actions of one’s will extirpate one’s iman. (First Volume, 163rd Letter)

The Qur’an al-karim says (what means):
(Those who believe in Allahu ta’ala and the Day of Resurrection do not like the enemies of Allahu ta’ala and His Prophet, even if they are their fathers, brothers, and relatives.) [Surat-ul-Mujadala 22]

(O you who believe! Do not take My enemies and yours as friends, and do not love them.) [Surat-ul-Mumtahina 1]

The following are stated in hadith-i sharifs:
(Three things intensify the flavor of iman: to love Allah and His Prophet more than everything else; to love for the sake of Allah a Muslim who does not love you, and not to love the enemies of Allah.) [Tabarani]

(The basis and the most dependable symptom of iman is hubb-i fillah and bughd-i fillah.) [Abu Dawud]

(Bughd means disliking and feeling hostility. Bughd-i fillah means disliking and feeling hostility for the sake of Allah. Its antonym, hubb-i fillah, means loving and making friends for the sake of Allah.)

(He who does not consider the enemies of Allah as enemies has not believed truly.) [I. Ahmad]

(He who loves the friends of Allah and considers His enemies as enemies has a perfect iman.) [Abu Dawud]

The sign of iman
Question: How can we know whether one has iman?
ANSWER
We cannot understand whether a person has iman simply by looking at their face. If they perform salat, then it can be inferred that they have iman. If one leaves a sin out of the fear of Allahu ta’ala, this is the sign of iman as well. Or becoming upset after committing a sin is a sign of iman, too. It is stated in a hadith-i sharif:

(He who becomes happy after doing good and who becomes unhappy after committing a sin is a true believer.) [Tirmidhi]

If one loves Muslims and dislikes enemies of Islam and bid’at holders, it is a sign of iman, too.

Likewise, loving Allahu ta’ala is a sign of iman. It is stated in a hadith-i sharif:
(Iman in the heart means loving Allahu ta’ala.) [Daylami]

Having iman is not sufficient. One has to have correct iman.
Loving Allahu ta’ala means considering His friends as friends and His enemies as enemies. The indication of iman is hubb-i fillah and bughd-fillah, which reveals whether one has iman or not. If a person loves pious Muslims and dislikes disbelievers and bid’at holders, it is understood that they have iman. At the same time, if they, too, obey prohibitions and commandments of Allahu ta’ala, there is not the slightest doubt anymore that they are Muslim.

Importance of sajdah
Question: If a disbeliever performs sajdah at-tilawah after listening to an ayah necessitating the performance of sajdah, can we say that they have become a Muslim?
ANSWER
Yes, we can say that they have become a Muslim. (Bazzaziyya, Ibn Abidin)

Likewise, he who performs salat in jama’at is considered a Muslim. In the true religions before Islam, salat would be performed not in jama’at, but individually. (Islam Ahlaki)

As the judgment is according to what is obvious and evident in our religion, a disbeliever is judged as a Muslim when they do what is mentioned above. However, they are not Muslim in reality unless they give up their false belief and have belief in the matters conveyed by our religion. Maybe they do it out of hypocrisy just to be seen as a Muslim. However, they are treated as a Muslim in worldly life unless something that discloses their disbelief is witnessed.

Performer of salat is a Believer
Question: Is an ahl-i bid’at who performs salat in jama’at called a disbeliever?
ANSWER
We cannot call a person about whose aqedah we have no knowledge a disbeliever because performing salat in jama’at is a symbol of Islam. If a person is seen while offering salat in jama’at, he is considered a Muslim unless his disbelief is known for certain because salat is a sign of belief.
Of course, there are exceptions to the rule. For example, a person, though he disbelieves, may come to the mosque to spy on Muslims in the mosque or with any other bad intention. However, we cannot call them a disbeliever as we do not know exactly who they are. Nobody is called a disbeliever on suspicion, without exact knowledge. One who denies an open principle of Islam, such as avoiding alcohol or covering oneself, falls out of the fold of Islam even if one performs salat.