Excellent praise and gratitude are due to Allah. May Allah increase the rank and honor of Prophet Muhammad ﷺ and protect his [Muslim] Nation from what he feared for it.
Allah the Exalted Said[1]:
﴿يَــٰأَيُّهَا ٱلَّذِينَ ءَامَنُواْ ٱتَّقُواْ ٱللهَ وَلتَنظُر نَفسٌ مَّا قَدَّمَت لِغَدٍ﴾
[which means] “O you who believed, fear Allah, and let each self look at that which it has prepared for tomorrow.”
^Aliyy [Ibn Abi Talib], may Allah accept his good deeds and honor his face, said, “Today is the work and tomorrow is the Reckoning.” This was narrated by Al-Bukhariyy in the Chapter of Ar-Riqaq.
The Greatest of Allah’s Rights over His Slaves
Know that the greatest of the rights of Allah the Exalted over His slaves is [confirming] His Oneness and that nothing would be associated with Him. This is because associating partners with Allah is the biggest sin committed by the slave. It is the sin that He does not Forgive [after death], and He Forgives whatever is less than that for whomever He Wills.
Allah the Exalted Said[2]:
﴿إِنَّ ٱللهَ لَا يَغفِرُ أَن يُشرَكَ بِهِ وَيَغفِرُ مَا دُونَ ذَلِكَ لِـمَن يَشَآءُ﴾
[which means] “Surely, Allah does not forgive the association of partners with Him, and He forgives whatever is less than that for whomever He Wills.”
Likewise, all types of blasphemy are not Forgiven by Allah [after death]. This is due to the Saying of Allah the Exalted[3]:
﴿إِنَّ ٱلَّذِينَ كَفَرُوا وَصَدُّواْ عَن سَبِيلِ ٱللهِ ثُمَّ مَاتُواْ وَهُم كُفَّارٌ فَلَن يَغفِرَ ٱللهُ لَهُم﴾
[which means] “Surely Allah will not Forgive those who blasphemed and those who blocked others from the path of Allah[4], and then died while being blasphemers.”
The Messenger of Allah ﷺsaid:
مَن شَهِدَ أنْ لا إِلهَ إِلا اللهُ وَحْدَهُ لا شَرِيكَ لَهُ، وأَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ وأَنَّ عِيسَى عَبْدُ اللهِ وَرَسُولُهُ وَكَلِمَتُهُ أَلْقَاهَا إِلى مَرْيَمَ، وَرُوحٌ مِنْهُ وَالجَنَّةَ حَقٌّ وَالنَّارَ حَقٌّ، أَدْخَلَهُ اللهُ الجَنَّةَ عَلَى مَا كَانَ مِنَ العَمَلِ
[which means] “Whoever testifies that no one is God except Allah alone without a partner, that Muhammad is His slave and His Messenger, that ^Isa (Jesus) is the slave of Allah and His Messenger, and the good news given to Mary, and a soul from Allah, that Paradise is true, and Hellfire is true, Allah will admit him to Paradise no matter what he did.” This hadith was narrated by Al-Bukhariyy and Muslim.
In another hadith:
فَإِنَّ اللهَ حَرَّمَ عَلَى النَّارِ مَنْ قَالَ لا إِلهَ إِلا اللهُ يَبْتَغِي بِذَلِكَ وَجْهَ اللهِ
[which means] “Surely Allah Made forbidden for Hell[5] whoever said no one is God except Allah sincerely for the sake of Allah.” This hadith was narrated by Al-Bukhariyy.
It is obligatory to associate the belief in the Message of Muhammad ﷺ with the testimony that no one is God except Allah. That is the least by which safety from the everlasting stay in Hell would take place.
[1] Surat Al-Hashr, ayah 18.
[2] Surat an-Nisa’, ayah 48.
[3] Surat Muĥammad, ayah 34.
[4] This means blocking others from embracing Islam.
[5] Meaning to stay in Hell forever, not a negation of a believer entering it.