In The Name of Allah the Most Merciful, The most Compassionate
The holy Qur'an was revealed in a society which was full of idols and idol worshipers. The verses which condemn invoking other than God relates to those idol worshipers. And truly it is shirk and it is unforgivable. ان الله لایغفر ان یشرک به و یفغر مادون ذلک لمن یشاء
What is Shirk and why does the holy Qur'an consider it unforgivable? To understand the answer of this question and to understand any other Quranic concept we should have a methodology. The holy Quran itself gives us this methodology:
Tadabbor: deep thinking about the verses, افلا یتدبرون القرآن ام علی قلوب اقفالها(محمد: 24)- کتاب انزلناه الیک مبارک لیدبروا آیاته(ص: 29
Referring back about Motashabihat to Mohkamat.
Now we begin to answer our question concerning the condemnation of shirk and being an unforgivable sin (of course without repentance). We want to reach to the answer through Tadabbor in the holy Quran. Is it possible to say that God has forbidden worshiping another entity because God is jealous and can tolerate no rival for himself? No, because being jealous is a bad attribute and the holy Qur'an four times announces that "All Good/Beautiful names Belong to Allah", for instance: و لله الاسماء الحسنی، فله الاسماء الحسنی
Is it possible to say that the reason of condemning shirk is: Shirk will cause a loss for Almighty Allah and will hurt Him? Again the answer is "no" because God is rich: غنی
So why is shirk an forgivable sin? It seems that because Shirk will lead to establishing another "authority" besides God's absolute authority. The meaning of authority is that Allah alone issues commandments for the life of humanity and the source of religious commandments is only the Almighty Allah. Shirk (a non-real source for commandments) provides the ground for violating Allah's Commandments forever for every human being and in every place because shirk means to believe in other gods, and this violation is not considered as "Sin" but is considered as a "law" like Allah's Law. So shirk will provide the ground for a constant violation of God's law under the title of "legal law" not under the title of "sin".
Consider this verse: "و کذالک زین لکثیر من المشرکین قتل اولادهم شرکاءهم". It means that the idol worshipers were killing their children for their Idols and this was not a sin in their belief but it was a rule or a legal action because they believed in another source for issuing commandments other than Allah.
Now another verse: "ام لهم شرکاء شرعوا لهم من الدین ما لم یأذن به الله (شوری: 21". As it is understood from this verse Shirk will lead to establishing another religion other than the religion of Allah.
So those who have conveyed Allah's law to mankind and have spent all their life to receive Allah's commandments through revelation and then convey them to mankind, they are best servants of Allah and their sayings are best among the sayings: "و من احسن قولا ممن دعا الی الله : فصلت : 33".
They did not establish commandments from their own so they are admired in the Holy Quran: "و انهم عندنا لمن المصطفین الاخیار ص:47" and many many verses that admire them.
Even Allah send salutation for them: "و سلام علی المرسلین صافات: 181" and salutation to the messengers.
They are bestowed with the rank of "Shafaah"(intercession). Shafaah (intercession) is a well known concept in the holy Qur'an and this high position is a gift from almighty Allah to His Messengers and to His friends اولیاء. Some of the Muslims belive that intercession is just in hereafter and many Muslims believe that it is also in Barzakh (after death and before hereafter) because these messengers and friends of God are alive like the martyrs, and their life after death is even stronger than in this worldly life, because they are nearer to Allah: "عند ربهم یرزقون - قی مقعد صدق عند ملیک مقتدر: قمر 55"
According to یونس : 3 it seems that intercession is not just for hereafter. Anyhow those who believe that invoking messengers, martyrs, friends of the Almighty Allah is in fact the very intercession that Allah bestowed to those who have been conveying Allah's law to human being can not be considered shirk at all and according to what i explained about shirk there is no any relationship between the concept of Shirk and believing in Shafaah (intercession) of good servants of the Almighty God. Shirk according to what I explained is applicable to those parliaments that pass laws against Allah's law, like legalization of homosexuality and let the people to believe in it as a legal law which can be followed by every one in every place and for every time.
In the end I would like to refer to a narration from Ahl-albayt that they said:
Bany Omayyad taught the people the Tauhid: monotheism but they did not say to them about shirk.
This means that obeying a king who issues commandments against Allah's law and consider them for the people as obligatory laws is shirk.