Doing Shirk Unknowingly
The forgiveness is not based on having / not having knowledge of the gravity of a particular sin, instead, it is based on person’s knowledge, resources and use of that knowledge and resources in a particular situation. Quran says: وَأَنزَلْنَا إِلَيْكَ الْكِتَابَ بِالْحَقِّ مُصَدِّقًا لِّمَا بَيْنَ يَدَيْهِ مِنَ الْكِتَابِ وَمُهَيْمِنًا عَلَيْهِ فَاحْكُم بَيْنَهُم بِمَا أَنزَلَ اللّهُ وَلاَ تَتَّبِعْ أَهْوَاءهُمْ عَمَّا جَاءكَ مِنَ الْحَقِّ لِكُلٍّ جَعَلْنَا مِنكُمْ شِرْعَةً وَمِنْهَاجًا وَلَوْ شَاء اللّهُ لَجَعَلَكُمْ أُمَّةً وَاحِدَةً وَلَـكِن لِّيَبْلُوَكُمْ فِي مَآ آتَاكُم فَاسْتَبِقُوا الخَيْرَاتِ إِلَى الله مَرْجِعُكُمْ جَمِيعًا فَيُنَبِّئُكُم بِمَا كُنتُمْ فِيهِ تَخْتَلِفُونَ Then [O Prophet!] We have revealed to you this Book with truth in corroboration of the Book before it, and standing as a guardian over it. Therefore, give judgement among them according to the guidance revealed by God, and do not yield to their whims by swerving from the truth revealed to you. For each of you, We have ordained a shariah that is a line of action. Had God pleased, He could have made of you one community: [but he did not do this] because He wanted to try you by what He has blessed you with . So, try to outdo one another in good deeds. To God shall you all [one day] return. Then He shall disclose upon you all your differences. (Quran 5:48) Shirk which is unforgivable is the one which a person commits consciously as Shirk. Therefore, if someone is knowingly committing Shirk after consciously reaching to this decision that Shirk is the only true religion, then that is the kind of shirk which is unforgivable, becasue there is no basis available to reach this conclusion and it would tantamount to establishing a lie/blame unto God. Secondly, this is not true that gravity of Shirk is not known to Humans as opposed to gravity of evil deeds. Quran makes it very clear that just like good and evil, monotheism had also been ingrained in human nature through ehd-e-Alast. Please refer to the video in the following link which includes a very insightful discussion on polytheism: Discussion 37854 • Reply 37863
Every individual will be given the opportunity to put forward his or her excuses. If the excuses are valid, God will give them concession, otherwise not. We read in the Quran in Surah Maida, that the misguided Christians who lost true guidance and believe in the Trinity can be excused, rather will be given favous, if they were truthful in their beliefs. See verses 5: 116 onwards.