Part 1: The Reality of The Tawheed Brought By The Messengers- [Sound Aqeedah is The Foundation of Upright Deeds]

Shaikh Saaleh Al-Fawzaan [may Allaah preserve him] said:

الحمد لله رب العالمين، وصلى الله وسلم على نبينا محمد خاتم الرسل، ومن تمسك بسنته وسار على نهجه الى يوم الدين

To proceed:

Indeed, [sound] Aqeedah is the foundation of the [upright] deeds and status of the of nations. The rectification of every nation and its [upright] ascendancy is tied to the soundness of its Aqeedah and Afkaar [Creed and Thoughts]; therefore, due to this reason, the Message of the Prophets [alayhimus salaatu was-salaam] came to rectify the Aqeedah [of the people]. So, the first thing every Messenger said to his people [was]:

ٱعۡبُدُواْ ٱللَّهَ مَا لَكُم مِّنۡ إِلَـٰهٍ غَيۡرُهُ

Worship Allah! You have no other Ilah (God) but Him. (La ilaha ill-Allah: none has the right to be worshipped but Allah).

[Surah Al-A’raaf’ Aayah 59]

[Allaah (The Most High) said]:

وَلَقَدۡ بَعَثۡنَا فِى ڪُلِّ أُمَّةٍ۬ رَّسُولاً أَنِ ٱعۡبُدُواْ ٱللَّهَ وَٱجۡتَنِبُواْ ٱلطَّـٰغُوتَ‌ۖ

And verily, We have sent among every Ummah (community, nation) a Messenger (proclaiming): “Worship Allah (Alone), and avoid (or keep away from) Taghut (all false deities, etc. i.e. do not worship Taghut besides Allah).”

[Surah An-Nahl’ Aayah 36]

And that is because Allaah [Glorified be He and free is He from all Imperfections] created the Khalq [i.e. Jinn and Mankind] to worship Him alone and not to associate anything as a partner with Him, just as [Allaah (The Most High)] said:

وَمَا خَلَقۡتُ ٱلۡجِنَّ وَٱلۡإِنسَ إِلَّا لِيَعۡبُدُونِ

And I (Allah) created not the jinns and humans except they should worship Me (Alone)

[Surah Adh-Dhaariyaat’ Aayah 56]

Source: Muhaadaraat Fil Aqeedah Wad-Da’wah’ vol 1′ pg 7. abridged and slightly paraphrased