Moreover, it is related that the Messenger of Allah (peace be upon him) said, "None of you (truly) believes until their desires are in accordance with what I have come with."
Based on that, a Muslim's Iman (Faith/belief) is not complete unless they believe in Allah, accept His Decree in minor as well as major issues and seek judgment from Shari'ah alone in all affairs whether related to the self, property or honor. Otherwise, a believer would be worshipping someone other than Allah, Who says:
And verily, We have sent among every Ummah (community, nation) a Messenger (proclaiming): "Worship Allâh (Alone), and avoid (or keep away from) Tâghût (all false deities i.e. do not worship Tâghût besides Allâh)."
So anyone who submits to Allah
(Exalted be He), obeys Him and seeks judgment from His Revelations is a true worshipper of Allah, while anyone who submits to other than Him and applies other law has indeed worshipped and followed Taghut (false gods). Allah (Exalted be He) 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 Tâghût (false judges) while they have been ordered to reject them. But Shaitân (Satan) wishes to lead them far astray.
Submission to Allah and renunciation of Taghut and their judgments are among the prerequisites of Shahadah (Testimony of Faith) that there is no god but Allah alone, without any partners, and that Muhammad is His servant and Messenger. Allah (Glorified be He) says:
Surely, His is the Creation and Commandment.
Since Allah is the Only Creator, He is the Only Commander (Glorified be He) and it is obligatory to obey His Commands.
Allah informs us about the Jews and Christians who have taken their rabbis and monks as lords besides Allah by obeying them in making lawful what is unlawful or unlawful what is lawful. Allah (Exalted be He) says:
They (Jews and Christians) took their rabbis and their monks to be their lords besides Allâh (by obeying them in things which they made lawful or unlawful according to their own desires without being ordered by Allâh), and (they also took as their Lord) Messiah, son of Maryam (Mary), while they (Jews and Christians) were commanded [in the Taurât (Torah) and the Injeel (Gospel)] to worship none but One Ilâh (God Allâh) Lâ ilâha illa Huwa (none has the right to be worshipped but He). Praise and glory be to Him (far above is He) from having the partners they associate (with Him).
It is related on the authority of ' Ady ibn Hatim (may Allah be pleased with him) how he thought that worshipping rabbis and monks can only be through slaughtering and vowing for them, prostrating and bowing before them and the like. This was clarified to him: "When he went to the Prophet (peace be upon him) to enter Islam and heard him reciting this Ayah, he said, 'O Messenger of Allah! We do not worship them.' He was referring to Christian monks, as he was a Christian before entering Islam. The Prophet (peace be upon him) said, 'Do you not follow them in deeming unlawful what Allah has made lawful and in deeming lawful what Allah has made unlawful?' He said, 'Yes, we do.' The Prophet (peace be upon him) said, 'That is worshipping them.'" (Related by Ahmad and AlTirmidhy who ranked it as Hadith Hasan [good Hadith])
A lHafizh Ibn Kathir said in his (Tafsir): "That is why Allah says: while they (Jews and Christians) were commanded [in the Taurât (Torah) and the Injeel (Gospel)] to worship none but One Ilâh (God Allâh) It means that only that which Allah forbids is the Haram (the forbidden and unlawful), and only that which Allah permits is the Halal (the permissible and lawful), whatever laws He laid down must be followed and His Judgments must be executed. The words: Lâ ilâha illa Huwa (none has the right to be worshipped but He). Praise and glory be to Him (far above is He) from having the partners they associate (with Him)." mean that Allah is Glorified and Exalted high above having partners, equals, helpers, rivals or children. There is no God but Him and He is the only Lord." (Vol. 2, p. 349)