He is outwardly Muslim but he rarely attends the mosque; should we think badly of him?
Question

                There is a man who claims to be a Muslim. He is from a European country and lives in a Muslim country. He rarely attends the mosque to pray, and when I say to him, “I do not see you observing the prayers on time,” he says that he prays at home. It should be noted that his residency visa and passport still show him as a Christian, but he says that he has a certificate which proves that he is a Muslim. What are our duties towards this man?.
Praise be to Allah.
    Those who claim to be Muslim should publicly practise the 
    rituals of Islam and adhere to its laws as much as they can. If a Muslim 
    claims that he is doing acts of worship such as prayer and giving zakaah, he 
    should be believed and what he says should be accepted. 

    All we know is what people show outwardly, and our Lord has 
    not commanded us to look into people’s hearts, because no one can do that. 

    The basic principle is that everyone who utters the 
    Shahaadatayn should be regarded as a Muslim, so long as he does not do 
    anything that nullifies his Islam. 

    It was narrated that Abu Hurayrah (may Allaah be pleased with 
    him) said: When the Prophet  (peace and blessings of Allaah be upon him) 
    died and Abu Bakr was appointed as caliph, and some of the Arabs reverted to 
    kufr, ‘Umar said: “O Abu Bakr, how can you fight the people when the 
    Messenger of Allaah  (peace and blessings of Allaah be upon him) said: 
    ‘I have been commanded to fight the people until they say Laa ilaaha 
    ill-Allaah (There is no god but Allaah), and whoever says Laa ilaaha 
    ill-Allaah, his wealth and his life are protected from me except in cases 
    dictated by Islamic law, and his reckoning will be with Allaah’?” Abu Bakr 
    said: “By Allaah, I will most certainly fight those who separate prayer and 
    zakaah, for zakaah is what is due on wealth. By Allaah, if they withhold 
    from me a young goat that they used to give to the Messenger of Allaah  
    (peace and blessings of Allaah be upon him), I will fight them for 
    withholding it.” ‘Umar said: “By Allaah, when I saw that Allaah had opened 
    Abu Bakr’s heart to the idea of fighting, I knew that he was right.” 

    Narrated by al-Bukhaari, 6924; Muslim, 20. 

    Al-Haafiz ibn Hajar (may Allaah have mercy on him) said: 

    Al-Khattaabi said: This hadeeth indicates that whoever 
    appears outwardly to be a Muslim, the outward rulings of Islam apply to him, 
    even if he is inwardly a kaafir. 
The difference of 
    opinion has to do with one whose corrupt beliefs are discovered and he shows 
    that he has recanted; can that be accepted from him or not? As for the one 
    whose position is unknown, there is no dispute that the outward rulings 
    apply to him. End quote. 

    Fath al-Baari, 12/279, 280 

    Since you do not know whether this man holds any corrupt 
    beliefs and he has not done anything that nullifies Islam, no one has the 
    right to accuse him of not being a Muslim. The basic principle is that he is 
    as he appears to be. 

    Read the following hadeeth and ponder the discussion that 
    took place between the Prophet  (peace and blessings of Allaah be upon 
    him) and Khaalid ibn al-Waleed (may Allaah be pleased with him): 

    It was narrated that Abu Sa’eed al-Khudri (may Allaah be 
    pleased with him) said: A man with sunken eyes, high cheekbones, a prominent 
    forehead, a thick beard, a shaven head and a waist-wrapper that was tucked 
    up, stood up and said: “O Messenger of Allaah, fear Allaah!” The Messenger 
    of Allaah  (peace and blessings of Allaah be upon him) said: “Woe to 
    you! Am I not the most entitled of all the people of earth to fear Allaah?” 
    Then the man turned away. Khaalid ibn al-Waleed said: “O Messenger of 
    Allaah, shall I strike his neck (cut off his head)?” He said, “No, for 
    perhaps he prays.” Khaalid said: “How many are there who pray and say with 
    their tongues what is not in their hearts?” The Messenger of Allaah  
    (peace and blessings of Allaah be upon him) said: “I have not been ordered 
    to search people’s hearts.” 

    Narrated by al-Bukhaari, 4351; Muslim, 1064 

    It was narrated that ‘Abd-Allaah ibn ‘Utbah said: I heard 
    ‘Umar ibn al-Khattaab (may Allaah be pleased with him) say: “People used to 
    be judged by the wahy (revelation) at the time of the Messenger of Allaah 
     (peace and blessings of Allaah be upon him), but now the wahy has 
    ceased. Now we will judge you according to what we see of your outward 
    deeds. Whoever appears good to us, we will trust him and draw close to him, 
    and what is in his heart has nothing to do with us. Allaah will call him to 
    account for what is in his heart. And whoever appears bad to us, we will not 
    trust him and we will not believe him, even if he says that inwardly he is 
    good.” Narrated by al-Bukhaari, 2641. 

    Undoubtedly prayer is the best of a person’s deeds, so if you 
    have seen him praying – even if that is only rarely – then he has shown you 
    that he is Muslim, so he should be trusted and believed. You should honour 
    this man and help him to apply the teachings of Islam and teach him the 
    rulings of sharee’ah, including the importance of prayers and of offering 
    prayer in congregation in the mosque. Show him the good attitude of Islam, 
    and if he is not sincere then perhaps he will be influenced and will become 
    sincere both outwardly and inwardly. 

    And Allaah knows best.