The one who doubts their Kuffr & Shirk...

From Ayaat such as that in which Allah says: 
"and He makes none to share in His Decision and His Rule" [18:26] 

It may be understood that the followers of those who promulgate laws other than those which Allah has ordained are committing shirk with Allah. This understanding is explained clearly in other Ayaat, such as the Ayaat which speaks of those who follow the laws of the shaytaan in permitting dead meat, claiming that it has been killed by Allah (and thus should be permitted): 
"Eat not (O believers) of that (meat) on which the Name of Allah has not been pronounced (at the time of the slaughtering of the animal), for surely it is Fisq. And certainly, the shayātīn (devils) do inspire their friends (from mankind) to dispute with you, and if you obey them [by making Al‑Maytah (a dead animal) legal by eating it], then you would indeed be mushrikūn" [6:121] 

Here Allah clearly states that they would be mushrikoon by obeying them. This is associating others with Allah in terms of obedience and following laws that are contrary to that which Allah has ordained — which is what is meant by worshipping the shaytaan in the Ayah in which Allah says: 
"Did I not command you, O Children of Ādam, that you should not worship shaytaan. Verily, he is a plain enemy to you. And that you should worship Me. That is the Straight Path" [36:60-61] 

And Allah tells us that His Prophet Ibrahim said: 
"‘O my father! Worship not shaytaan. Verily, shaytaan has been a rebel against the Most Gracious (Allah)’" [19:44]

And Allah says: 

"They (all those who worship others than Allah) invoke nothing but female deities besides Him (Allah), and they invoke nothing but shaytaan, a persistent rebel." [4:117] 

They do not worship anything but a devil, i.e., by following his laws. Hence Allah calls those whom they obey in the sins that they made attractive to them “partners”, as in the Ayah: 

"And so to many of the mushrikoon their ‘partners’ have made fair-seeming the killing of their children, in order to lead them to their own destruction and cause confusion in their religion" [6:137] 

The Prophet (saws) explained this to ‘Adiyy ibn Hatim (ra) when he asked him about the verse in which Allah says: 

"They (jews and christians) took their rabbis and their monks to be their lords besides Allah" [9:31] 

He explained that they permitted to them that which Allah has forbidden and they forbade to them that which Allah has permitted, and they (the people) followed them in that; that is how they took them as lords. One of the clearest proofs concerning that is the fact that in Sūrat an-Nisa’, Allah mentioned those who want to refer for judgement to something other than the Laws prescribed by Him and wonders at their claim to be believers, because their claim to be believers even though they want to refer for judgement to false judges shows that they have reached a level of lying which is astounding. That is what Allah says: 

"Have you not seen those (hypocrites) who claim that they believe in that which has been sent down to you, and that which was sent down before you, and they wish to go for judgement (in their disputes) to the taghout (false judges) while they have been ordered to reject them. But shaytaan wishes to lead them far astray" [4:60] 

And with these Heavenly texts that we have mentioned, it becomes quite clear that the ones who follow the fabricated laws, which the shaytaan has legislated upon the tongues of his ‘Awliya and which oppose that which Allah has legislated upon the tongues of His Messengers that no one doubts their Kuffr and their Shirk except him who Allah has removed his sight and has blinded them to the light of the revelation. 

Take note. Know that it is wajib to differentiate between the invented institutions, which are Kuffr in the Creator of the Heavens and Earth, to judge according to them and between the institutions (like traffic lights), which aren’t. 

As for the legislative institutions, which contradict the legislation of the Creator of the Heavens and the Earth, then judging with these is Kuffr in the Creator of the Heavens and the Earth. Such as saying that the preference of the males over the females in the inheritance is unjust and that is should be that they are equal in inheritance and like the saying that polygamy is thulm and that divorce is thulm against the women and that stoning and cutting off the hand and things like this are barbaric acts which should not be carried out against the people and things like that. So ruling by institutions such as these upon individuals and the society and their wealth and their property and minds and Deen is Kuffr in the Creator of the Heavens and the Earth and it is a rebellion against the Law of the Heavens, which was given by the Creator of all the creation and He is more knowledgeable of the benefits towards His creation than to have another legislator along with Him. 

"And do they have partners who have legislated in the Deen what Allah did not give permission for?" 

[Adwa’ Al Bayan, Tafseer of Suratul Kahf, By al-‘Allamah Muhammad al-Ameen ash-Shanqiti (rahimahUllah)]