Definition: the scholarly foundations refer to the holistic matters that the Da’wah are interested in.
The first and most important foundation is the Tawheed (i.e. the belief in the oneness of Allah in worship)
Allah send all the Prophets to assert this meaning as He, the Exalted, says:
وَاسْأَلْ مَنْ أَرْسَلْنَا مِن قَبْلِكَ مِن رُّسُلِنَا أَجَعَلْنَا مِن دُونِ الرَّحْمَنِ آلِهَةً يُعْبَدُونَ
“And ask (O Muhammad) those of Our Messengers whom We sent before you: ‘Did We ever appoint ‘aaliha (gods) to be worshipped besides the Most Beneficent (Allah)?'” (az-Zukhruf: 45).
He Exalted be He, also said:
وَمَا أَرْسَلْنَا مِن قَبْلِكَ مِن رَّسُولٍ إِلَّا نُوحِي إِلَيْهِ أَنَّهُ لَا إِلَهَ إِلَّا أَنَا فَاعْبُدُونِ
“And We did not send any Messenger before you (O Muhammad) but We inspired him (saying): ‘Laa ilaaha illaa Anaa [none has the right to be worshipped but I (Allah)], so worship Me (Alone and none else).'” (al-Anbiyaa: 25); and this is the repeated say of each prophet “Worship Allah, for you have no other (true) god but Him”. Indeed, this is what Hood, Saalih, Ibraaheem, Moosaa, ‘Eesaa, and every other prophet said to their people: “Worship Allah, for you have no other (true) god but Him”.
Accordingly, the mission of the Prophets as well as the Callers is one; and this should be the only cause of any Call which claims that it is upon the Truth and Sunnah and that it is the Saved Sect; it is the cause of Tawheed which means asking the people to worship Allah and nothing but Allah.
In this respect, Rib’ee Ibn ‘Aamir—may Allah be pleased with him–, who understood this meaning from the Prophet—peace be upon him– , answered Rustum, the then leader of the Persian army, when the latter asked him about what the Prophet— صلى الله عليه وسلم —came with; he (Rib’ee) said:
إن الله أبتعثنا لنخرج من شاء من عبادة العباد إلى عبادة الله،
ومن ضِيْقِ الدنيا إلى سعتها، ومن جور الأديان إلى عدل الإسلام
“Allah send us to save those He willed from the worship of worshippers and take them into the worship of Allah, from the hardship of the worldly life into its ease, and from the injustice of religions into the justice of Islam”.
Thus, the task of all the prophets and their followers is to save people from the null and void worship of false deities and take them into the worship of Allah, Exalted be He, make them true worshippers of Allah, teach them about Tawheed, and teach them about Allah; and this is the cause for which Allah created all His creation as He, the Exalted, says:
وَمَا خَلَقْتُ الْجِنَّ وَالْإِنسَ إِلَّا لِيَعْبُدُونِ
“And I (Allah) created not the jinns and humans except they should worship Me (Alone)” (adh-Dhaariyaat: 56).