Praise be to Allah.
To prove that a person is a Muslim outwardly and is to be treated like other Muslims, it is sufficient first of all for him to have uttered the Shahaadatayn (twin declaration of faith).
Allah, may He be exalted, says (interpretation of the meaning):
“O you who have believed, when you go forth [to fight] in the cause of Allah , investigate; and do not say to one who gives you [a greeting of] peace ‘You are not a believer,’ aspiring for the goods of worldly life; for with Allah are many acquisitions. You [yourselves] were like that before; then Allah conferred His favour upon you, so investigate. Indeed Allah is ever, with what you do, Acquainted”
It was narrated that Usaamah ibn Zayd (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) sent us on a campaign to al-Huruqah, and in the morning we attacked the people and defeated them. An Ansaari man and I caught up with one of their [the enemy’s] men and when we overpowered him he said, Laa ilaaha ill-Allah. The Ansaari man held back, but I stabbed him with my spear and killed him. When we came, he [the Ansaari] told the Prophet (blessings and peace of Allah be upon him) about that and he said: “O Usaamah, did you kill him after he said Laa ilaaha ill-Allah?” I said: He was only trying to protect himself. He [the Prophet (blessings and peace of Allah be upon him)] kept repeating it until I wished that I had not become Muslim before that day.
Narrated by al-Bukhaari (4269) and Muslim (96).
Al-Qurtubi (may Allah have mercy on him) said:
If the Muslim encounters a disbeliever who does not have a covenant with the Muslims, it is permissible for him to kill him [in a war situation in which his people are at war with the Muslims] but if he says Laa ilaaha illa Allah, it is not permissible to kill him, because he has protected himself with the shield of Islam, which means that his life, his property and his wealth must be safe, and if the Muslim kills him after that, he is to be executed in return. Rather those referred to in these reports were not executed because that was at the very beginning of Islam, and they thought that he had said it in order to save himself and for fear of the sword, and that the only way a person could protect himself in this situation was if he said the shahaadah willingly and with conviction. But the Prophet (blessings and peace of Allah be upon him) explained that he was protected regardless of the reason why he said it, hence he said to Usaamah: “Did you open his heart to find out whether he said it [out of conviction] or not?” Narrated by Muslim. In other words, did you open his heart to see whether he was sincere when he said it or was lying? That is not possible, so there is no option left except to take his words at face value.
This leads us to an important principle in the field of fiqh, which is that rulings are to be based on what is likely to be the case and how things appear to be, not on what is definitely the case and what is in the heart.
End quote from Tafseer al-Qurtubi (7/51).
After uttering these words, the individual should demonstrate that he is adhering to the requirements of this word of Tawheed, so he should show that he is committed to it and is adhering to Islamic teachings.
It was narrated from Ibn ‘Umar that the Messenger of Allah (blessings and peace of Allah be upon him) said: “I have been commanded to fight the people until they bear witness that there is no god except Allah, and that Muhammad is the Messenger of Allah, and they establish regular prayer and pay zakaah. If they do that, then their blood and wealth are protected from me except in cases dictated by Islamic law, and their reckoning will be with Allah.”
Narrated by al-Bukhaari (25) and Muslim (22).
It was narrated that Anas ibn Maalik said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “I have been commanded to fight the people until they say Laa ilaaha ill-Allah. If they say it, and pray as we pray, and face towards our qiblah, and slaughter meat as we do, then their lives and wealth are protected from us, except in cases dictated by Islamic law, and their reckoning will be with Allah.” Narrated by al-Bukhaari (392).
According to another version that was narrated by al-Bukhaari, it was narrated that Anas ibn Maalik said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever prays as we pray, faces towards our qiblah, and eats meat slaughtered by us, he is a Muslim who comes under the protection of Allah and the protection of His Messenger, so do not transgress against the protection granted by Allah.” Al-Bukhaari (391).
Al-Baghawi (may Allah have mercy on him) said:
This hadith indicates that the way people deal with one another should be based on what they appear outwardly to be, and not based on what they have in their hearts. The one who complies with the rituals of Islam [and appears to be a practising Muslim] is to be deemed a Muslim and is subject to the rulings of Islam, and what is in his heart is not to be inquired into.
End quote from Sharh as-Sunnah (1/70).
It should be understood that outward signs of faith are not always indicative of faith in the heart.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
Outward appearance of faith, which forms the basis for which one may be deemed Muslim and subject to the rulings of Islam in this world, does not necessarily mean that an individual is a believer in his heart, on the basis of which a person will be among the people of bliss in the hereafter, for the hypocrites who said “‘We believe in Allah and the Last Day,’ but they are not believers” [al-Baqarah 2:8] outwardly appeared to be believers who prayed with the people, and they fasted, went on Hajj, and went out on campaign, and the Muslims intermarried with them and inherited from them. The hypocrites at the time of the Messenger of Allah (blessings and peace of Allah be upon him) were not deemed by the Prophet (blessings and peace of Allah be upon him) to be disbelievers, and he did not apply the same rulings to them as he did to those who were openly disbelievers, with regard to marriage, inheritance and so on.
End quote from Majmoo‘ al-Fataawa (7/210).
In order for a person to be truly Muslim, both outwardly and inwardly, he must fulfil all the conditions of the word of Tawheed.
This has been explained previously in the answers to questions no. 12295 and 9104.
Shaykh Sulaymaan ibn ‘Abdillah ibn Muhammad ibn ‘Abd al-Wahhaab (may Allah have mercy on him) said:
It was narrated that ‘Ubaadah ibn as-Saamit said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever bears witness that there is no god but Allah alone, with no partner or associate, and that Muhammad is His slave and His Messenger, and that ‘Eesa (Jesus) is His slave and His Messenger, and His Word that He bestowed upon Maryam and a soul created by Him, and that Paradise is true, and that Hell is true, Allah will admit him to Paradise, whatever his deeds may be.”
The words “Whoever testifies that there is no god but Allah” mean: Whoever utters these words, understanding their meaning and acting in accordance with them both inwardly and outwardly, as indicated in the verses (interpretation of the meaning):
“So know, [O Muhammad], that there is no deity except Allah”
“but only those who testify to the truth [can benefit], and they know”
As for uttering these words without knowing what they mean or acting in accordance with them, that is not beneficial, according to scholarly consensus.
End quote from Tayseer al-‘Azeez al-Hameed (p. 51).
With regard to these conditions, what is required of us is to understand their meanings and implications, and to believe in those meanings.
As for memorizing these conditions and knowing the scholarly definitions and classifications thereof, that is not necessary. Rather testing people on that is an innovation that is to be rejected. What is required is to teach people their religion and their beliefs (‘aqeedah) and to think positively of them.
Most ordinary Muslims are not able to list these conditions, but they have those concepts in their minds. If an ordinary Muslim is asked about the obligation to love Allah, may He be exalted, and His Prophet (blessings and peace of Allah be upon him), and the rulings on obeying them, accepting what they tell us and believing in that and all other conditions of the word of Tawheed, he will confirm all of that.
The sign of that is that we see them adhering to obligatory duties and being keen to do so, without being forced or being afraid of anyone, and we see them going and asking scholars about what they do not know of rulings, and other signs that are indicative of their sincere commitment to Islam.
Shaykh Haafiz al-Hakami (may Allah have mercy on him) said:
The one who says Laa ilaaha ill-Allah will not benefit from merely uttering these words, unless he learns and complies fully with these seven conditions. What is meant by complying fully with them is when a person understands them all and adheres to them all, without thinking or doing anything that is contrary to any of them.
What is meant by learning them is not merely listing the phrases and memorizing them. How many ordinary (relatively uneducated) Muslims understand them and comply fully with all of them, and adhere to them, even though if they were told to list them, they would not be able to do so.
And how many there are who have memorized them and are able to list them readily, but you see them doing things that are contrary to them. Guidance is in the hand of Allah, and it is Allah Whose help we seek.
End quote from Ma‘aarij al-Qubool (2/418).
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him) said:
What is required of all Muslims is to fulfil this word of Tawheed by paying attention to these conditions. Once the Muslim understands the meaning of this word and adheres to it, he is a Muslim whose life and wealth are sacred, even if he does not know these conditions in detail, because what matters is to learn the truth and act upon it, even if the believer does not know the details of the conditions required.
End quote from Majmoo‘ Fataawa ash-Shaykh Ibn Baaz (7/58).
We may say something similar about all other issues of ‘aqeedah.
Once it is established that a person should be judged on the basis of what he outwardly shows of faith, it should be understood that checking on faith in the heart and judging people’s beliefs, the true nature of their religious commitment and the level of their sincerity, is not something for people to do; rather that is for Allah, the Lord of the Worlds, Who will judge His slaves on the basis of His knowledge, for “Indeed, your Lord will judge between them by His [wise] judgement. And He is the Exalted in Might, the Knowing” [an-Naml 27:78].
The Messenger of Allah (blessings and peace of Allah be upon him) said: “I have not been commanded to check what is in people’s hearts or minds.” Narrated by al-Bukhaari (4351) and Muslim (1064).
With regard to people such as this man whom you described, in principle what is most likely to be the case is that they have faith and belief in general terms, which one hopes will be sufficient to save them on the Day of Resurrection.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
Most people, if they become Muslim after having been disbelievers, or they are born in Islam and adhere to its teachings, and if they are people who obey Allah and His Messenger, then they are Muslims and they believe in general terms, but for true faith to enter their hearts, that is something that only happens little by little, if Allah wills to grant them that. Otherwise, many people do not attain the level of certain faith or of striving for Allah’s sake, and if anyone tries to spread doubts then they will start doubting, and if they were enjoined to strive in jihad they would not do so. They are not disbelievers or hypocrites; rather they do not have enough faith and knowledge to ward off whatever doubts occur to them, and they do not have such strong love for Allah and His Messenger that they would give it precedence over their love of family and wealth. If these people are spared from tests and die, they will enter Paradise.
End quote from Majmoo‘ al-Fataawa (7/271).
And Allah knows best.