Bismillah ar-rahman ar-rahim
Tawhid or Monotheism is the most fundamental concept in Islam and holds Allah (God) as one (wāḥid) and unique (aḥad). Any good deed, be it salat, sawm, zakat, hajj, infaq, birrul walidain (dutiful to parents), etc. is not possible to be accepted by Allah if one rejects tawhid. Allah says in the Qur'an:
“And whoever does righteous deeds, whether male or female, while being a believer (Mu'min) - those will enter Paradise and will not be wronged, [even as much as] the speck on a date seed.” (An-Nisa: 124)
“If you should associate [anything] with Allah , your work would surely become worthless, and you would surely be among the losers.” (Az-Zumar: 65)
Scholars of ‘Aqeeda divide tawhid into three aspects:
Tawhid Ar-Rububiyyah -the tawhid of Lordship. It pertains to Allah’s right as the Lord, and all that entails–the Creator, the Sustainer.
Tawhid Al-Uloohiyyah -the tawhid of worship. It pertains to Allah’s right to be worshipped, without any partners.
Tawhid Al-Asmaa wal Sifat -the tawhid of Allah’s names and attributes–such as that he is Ar-Rahman and Ar-Rahim.
Denying part or whole of any of the parts of any of these three is shirk.
It is said that Shaikh Muhammad ibn 'Abdul-Wahhaab gave attention to Tawhidul-Uloohiyyah in his time when he saw that the people fell short of Tawhid in that aspect, and that Imaam Ahmad in his time gave attention to Tawhidul-Asmaa was-Sifaat when he saw the people falling short of Tawhid in that aspect.
But as for today then the people fall short with regard to another type of tawhid, that is,
Tawhidul-Haakimiyyah -That judgement/decree is for Allaah alone.
so therefore we must give attention to it. It demands takfir of the rulers who had rejected the Haakimiyyah of Allaah by not ruling by what Allaah has revealed, and also takfir of those ruled over (i.e. civilians), when they became satisfied with this.
As the Holy Qur'an says,
"Those who do not govern by what Allah has revealed are disbelievers (al-kafiroon)"
[Al-Quran, Surah Maidah, Ayah 44]
Wa Akhiru Dawana Anilhamdulillahe Rabbil A'lameen