Who is a Believing Muslim
Whoever knows Allah and His Messenger and uttered the shahadah even once in his life, and he accepted that with conviction, is a believing Muslim. Whoever knew that and uttered that, but he did not have conviction, is not a Muslim and not a believer to Allah. As for us, he is a Muslim, because his inner situation is hidden from us. If he only shows Islam outwardly and he dislikes Islam inwardly, or is doubtful in his heart whether Islam is correct or not, then he is a hypocrite blasphemer and he is included in the Saying of Allah the Exalted,
﴿إِنَّ الْمُنَافِقِينَ فِي الدَّرْكِ الْأَسْفَلِ مِنَ النَّارِ وَلَن تَجِدَ لَهُمْ نَصِيرًا﴾
[which means] “Certainly the hypocrites will be in the very bottom of Hell.” So he and the damned blasphemer will both be forever and ever in Hell. What some have said, that the faith of the blasphemer is valid without utterance[1], while able, is an invalid saying. Some said, whoever was raised among Muslim parents, it is enough for him to know and have the conviction about the validity of his Islam and Iman even if he did not utter it once. Furthermore, whoever had a valid Iman and Islam is safe from staying in Hell forever, even if he did not perform any of the obligatory acts, like the five prayers and the fasting of Ramadan, and he did not avoid the sins, and he died in this situation before repentance. Allah will Pardon a portion of those and Make them enter Paradise without torture, and He will torture a portion of them, then Make them come out of Hell and admit them into Paradise, and Allah Knows best about whom He will Pardon and whom He will not pardon. As for the one who died after repentance, and he fulfilled all of what Allah Made obligatory upon him, and he avoided the forbidden matters, then it is as if he did not sin, because of what the Prophet ﷺ said,
التَّائِبُ مِنَ الذَّنْبِ كمَن لا ذنْبَ لهُ
[which means] “The one who repents from the sin is like the one who has no sin.” This is an authentic hadith narrated by Ibn Majah from the route of Ibn Mas^ud.
It is narrated in Sahih Al-Bukhariyy that a man said, “Oh Messenger of Allah, should I embrace Islam or should I fight?” He said, “Embrace Islam, then fight.” So he embraced Islam, he fought, and he was killed. The Messenger of Allah ﷺ said, “He did little and was rewarded a lot,” meaning that he reached the status of a martyr after Islam eradicated every sin that he did previously. So the great merit is for Islam, because had he not embraced Islam, no deed that he did would have benefited him. This man was among the fighters because his people, who were Muslims, went out to fight, but he had not embraced Islam. Then Allah inspired him to ask the Messenger, so he asked, and so the Messenger guided him to embrace Islam and then to fight.
[1] i.e., of the Shahadah.