The Greatest Right of Allah upon His Slaves
Know that the greatest right of Allah upon His slaves is to clear Him from resemblance to the creation and to not associate anything with Him. Associating partners with Allah is the most enormous sin a slave can commit; it is the sin that Allah does not forgive but forgives what is lesser to whomsoever He willed. Allah said in Surah an-Nisa’,48:
{إِنَّ اللَهَ لَا يَغْفِرُ أَن يُشْرَكَ بِهِ وَيَغْفِرُ مَا دُونَ ذَٰلِكَ لِمَن يَشَاءُ ۚ وَمَن يُشْرِكْ بِاللهِ فَقَدِ افْتَرَىٰ إِثْمًا عَظِيمًا} [سورة النساء – آية ٤٨].
which means: [Surely, Allah does not forgive that anything be associated with Him and forgives what is besides that to whomsoever He wills. The one who associates partners with Allah has committed the greatest sin ]
Explanation
Knowing Allah while exclusively worshipping (the utmost submission to) Him is the greatest right of Allah upon His slaves. The most enormous sin a slave commits is blasphemy. Blasphemy is of two kinds–blasphemy involving associating partners with Allah and blasphemy involving other than associating partners with Him. Every association of partners with Allah is blasphemous but not every blasphemy is an association of partners with Allah. Henceforth, the greatest right of Allah upon His slaves is to worship Him and to not associate anything with Him. Allah, the Exalted, informed that He forgives all sins for whomsoever He willed among His Muslim worshippers who do not liken Allah to the creations and who clear Him from non-befitting attributes so long as they avoid both kinds of blasphemy–associating partners with Allah by worshipping others or the blasphemy of other than associating partners with Allah, like belying the Messenger of Allah and belittling Allah or His messenger. What also elucidates this same matter is the saying of the Prophet, sallallahu ^alayhi wa sallam,narrated by Imam Ahmad:
إن الله ليغفرَ لعبده ما لم يقع الحجاب: قالوا وما وقوع الحجاب يا رسول الله؟ قال أن تموت النفس وهو مشركة
which means: <<Allah forgives His slave so long the veil does not fall. The listeners asked, ‘What is the falling of the veil?’ He said, ‘For a soul to die engaged in (blasphemy of) associating partners with Allah’.>>
Hence, blasphemy in all of its kinds, is the one sin that Allah does not forgive for one; that is, for one who remains in it until death or until one is certain to die as in seeing the Angel of Death and the angels of torture or in the situation of definite drowning and the like, for this is annexed to death.
In conclusion, blasphemy is not forgiven except by one embracing Islam at the time it is still accepted from him. Therefore, if one embraces Islam after the acceptable time lapses, his doing so does not erase his blasphemy. Blasphemy is the most enormous of all sins, after which comes manslaughter that Allah forbade unless one is killed lawfully. The saying of Allah in Surah alBaqarah, 191:
وَالْفِتْنَةُ أَشَدُّ مِنَ الْقَتْلِ
means: [Associating partners with Allah is more severe than manslaughter.] Blasphemy is the gravest of all injustice as Allah clearly stated in Surah Luqman, 13:
إنَّ الشِّرْكَ لَظُلْمٌ عَظِيمٌ
and in Surah ِAl-Baqarah, 254:
وَالْكَافِرُونَ هُمُ الظَّالِمُونَ