The correct 'Aqeedah is the foundation of Islam and the foundation stone of the Islamic nation. It is known through evidences of the Sharee'ah from the Book and the Sunnah, that one's actions and statements can only be correct and acceptable to Allaah سبحانه و تعالى when they come up from a correct 'Aqeedah. If, however, the 'Aqeedah is not correct then the actions and statements that arise due to it are invalid. Allaah سبحانه و تعالى the Exalted says, "……And whoever denies the faith - his work has become worthless, and he, in the Hereafter, will be among the losers. Qur’aan.surah Maidah 5:5 "And indeed it has been revealed to you [O Muhammad] as it was to those before you [saying]: 'If you join others in worship with Allaah [then] surely [all] your deeds will be in vain and you will certainly be amongst the losers.” Qur’aan.Surah Zumar 39:65 [a]
Al-Aqeedah linguistically is derived from the term Aqada. In Arabic, one states, "Aqada the rope" when the rope is tied firmly. And Allaah سبحانه و تعالى says in the Qu'raan, "And as for those whom your right hands have made a covenant (Ar., aqadat)" [Al-Nisa 33].
And Allaah سبحانه و تعالى also says, "But He will take you to task for the oaths which you swear in earnest (Ar., aqadtum) [Al-Maida, 89] which means asserted and adhered to, as proven in the ayath, "And break not oaths after the assertion of them" [An-Nahl, 91].
If one says, "Aqadtu such and such," it means his heart is firm upon such and such.
Therefore, al-aqeeda or al-itiqad according to the scholars of Islam is: The firm creed that one's heart is fixed upon without any wavering or doubt. It excludes any supposition, doubt or suspicion.
It is the belief in Allaah سبحانه و تعالى, His Angels, His Books, His Messengers, the Last Day and belief in al-Qadar (Predestination) - it’s good and evil. These are called the Pillars of Eeman (faith). 
According to Dr. Abdul Aziz Al-Qari رحيمه الله:
Al-Aqeedah linguistically is derived from the term aqada. In Arabic, one states, "Aqada the rope" when the rope is tied firmly. And, "Aqada the sale" or "He settled the sale" when the person ratifies and contracts a sale or agreement. And Allaah سبحانه و تعالى says in the Qur'aan,
"And as for those whom your right hands have made a covenant (Ar.,aqadat)" [Al-Nisa 33]. And Allaah سبحانه و تعالى also says,
"But He will take you to task for the oaths which you swear in earnest (Ar., aqadtum)" [al-Maida, 89] which means asserted and adhered to, as proven in the verse,
"And break not oaths after the assertion of them" [an-Nahl, 91]. If one says, "Aqadtu such and such," it means his heart is firm upon such and such.
Therefore, al-aqidah or al-itiqad according to the scholars of Islam is: The firm creed that one's heart is fixed upon without any wavering or doubt. It excludes any supposition, doubt or suspicion. 
a. Beliefs are issues, which are not related to how an act is performed, like belief in the Rububiyah (Lordship) of Allaah سبحانه و تعالى, the obligation to worship Him (alone), and the belief in the rest of the aforementioned pillars of Eeman. These are called Asliyah - the basic foundation.
b. Actions are issues related to how actions are performed like Salaat (prayer), Zakaat (charity) and Sawm (fasting) and other rulings with regards to actions. These are termed as Far'eyyah - the branches, because their soundness or corruption is based upon the beliefs. 
The Correct Aqeedah (belief) is the foundation upon which the religion is based and with it, the actions are set aright, as the Most High, said, 'So, whoever hopes for the meeting with his Lord, let him work righteousness and associate none as a partner in the worship of his Lord.' [Surah al-Kahf (18): 110]
And indeed, it has been revealed to you (O Muhammad), as it was to those (i.e., Prophets) before you, 'If you join others in worship with Allaah, (then) surely (all) your deeds will be in vain, and you will certainly be among the losers.' [Surah az-Zumar (39): 65]
These aayaath, and the numerous narrations that have been related concerning their meaning confirm that actions are not accepted unless they are free from Shirk (polytheism). Therefore, it was the main concern of the Messengers (Peace be upon them all) to rectify the beliefs first and thus, the first thing, they called their nations to, was the sole worship of Allaah سبحانه و تعالى and abandonment of worship to anything other than Him. As He, the Most High, says, 'And verily, We have sent among every Ummah (community, nation) a Messenger (proclaiming), 'Worship Allaah (Alone), and avoid the Taghoot (everything that is worshiped other than Allaah).' [Surah An-Nahl (16): 36]
Narrated Ibn `Abbas: When the Prophet ﷺ sent Mu`adh to Yemen, he said to him, "You are going to a nation from the people of the Scripture, so let the first thing to which you will invite them, be the Tauhid (Tawheed) of Allaah سبحانه و تعالى. If they learn that, tell them that Allaah سبحانه و تعالى has enjoined on them, five prayers to be offered in one day and one night. And if they pray, tell them that Allaah سبحانه و تعالى has enjoined on them Zakat of their properties and it is to be taken from the rich among them and given to the poor. And if they agree to that, then take from them Zakat but avoid the best property of the people." Sahih Al Bhukari Vol 9:469 (NE 7372) 
The first thing that every Prophet addressed his people with was, 'Worship Allaah ! You have no other Ilah (deity worthy of being worshiped) but Him.' [Surah al-A'raf (7): 59, 65, 73, 85]. It was said by Nuh, Hood,Saleh,Suhaib and all the other Prophets (Peace be upon them all) to their nations.
The Messenger of Allaah ﷺ stayed in Makkah for thirteen years, after (he was bestowed with) Prophet-hood, calling people to Tawheed, and rectifying their Aqeedah, because it is the foundation upon which the entire structure of the Deen stands.
The ’Aqeedah (creed) must come first because it is the foundation of the da’wah (call) of the Prophets (Peace be upon them all) as Allaah سبحانه و تعالى the Exalted said upon the tongue of every Prophet He sent to his people: “O people! Worship Allaah, you do not have a deity other than Him.” [Surah Hood 11:84]
The ’aqeedah (creed) must come first because it is the decisive partition between abiding in the Fire (forever) and being saved from it. Have you not heard the statement of Allaah سبحانه و تعالى – Sublime is His Affair: “Verily Allaah does not forgive that shirk (association) be made with Him, and He forgives whatever is less than that for whomever He wishes. And whoever commits shirk with Allaah, then he has indeed perpetrated a great sin.” [Suratun-Nisaa‘ 4:48]
The ’aqeedah must come first because shirk with Allaah سبحانه و تعالى is the greatest form of oppression as Allaah سبحانه و تعالى the Exalted says: “Verily Shirk is indeed a great transgression.” [Surah Luqmaan 31:13]
The ’aqeedah must come first because the only real cause for the differing and destruction that has come upon our Islamic world is the purified ’aqeedah. So indeed the creed of many of the Muslims today has become corrupted by matters of greater shirk from making tawaaf (encircling) the graves, and seeking help from the dead people inside of them, and trying to fulfill ones needs and affairs from them. So they return to the first Jaahiliyyah (pre-Islamic times of ignorance), and it is as if they have not heard the statement of Allaah سبحانه و تعالى the Exalted:
‘‘Verily those whom you call upon besides Allaah do not hear your calls. And if they were able to hear you, then they would not be able to answer you. And upon the Day of Judgement, they will disown your worship of them, and there is none like He who is the All-Knower.” [Suratul-Faatir 14] 
[a] The Correct Islamic 'Aqeedah And That Which Opposes It by 'Abdul-'Azîz Ibn Bâz رحيمه الله , http://www.Islam.net/main/display.php?category=73&id=37