Belief in Allah, the one free from all defects, is to believe that He is the only true deity deserving of worship, as opposed to anything else that is worshipped. This is because he is the sole creator of the slaves, the granted of kindness and goodness to them, the one who takes charge of their provision and he is fully aware of both their secrets and that which they openly manifest. He is the one who rewards those who obey him and punishes those who disobey him. Allah say:
“and I did not create jinn and mankind except to worship me alone. I seek no sustain acne from them nor that they should feed me. Verily Allah is the Provider, Lord of Power, the Strong” (51:56-58)
“O mankind! Worship your lord who created you and those who were before you so that you may become pious. The one who has made the earth as arresting place for you and the sky as a canopy, sent down rain from the sky and brought forth fruits their with as a provision for you. Then do not set up rivals to him while you know (that there is nothing similar to him” (2:21-22)
He sent Messenegers and Books in order to explain the reality of worshipping him alone, to call to it and warn others from it.
“And verily, we have sent among every ummah ( community, nation) a messenger, proclaiming worship Allah alone and avoid taghut (everything worshipped besides Allah)” (16:36)
“and we did not send any messenger before you (O Muhammad) except we inspired him (saying): ‘La ilaha ilaa Anaa (none has the right to be worshipped but I (Allah). So worship Me (Alone and none else)” (21:2)
“This is a book whose verses are perfected (in every sphere of knowledge) and then explained by one who is all wise and well acquainted (with all things). (It says) worship none but Allah. Verily, I (Muhammad) am unto you one who warns and and brings glad tidings from him” (11:1-2)
Most of Quran was sent for this great principle.
“and your lord has decreed that you worship none but him” (39:2-3)
“so worship Allah, making the religion purely for him, even though the unbelievers hate it” (40:14)
Belief in Allah also includes belief in everything Allah has made obligatory on his slave. Such as the five pillars of islam. Which are testifying that one has the right to be worshipped except Allah and that Muhammad (sallalahu alayhi wasallam) is the messenger of Allah, the prayer (salah), giving the charity (zakat), fasting in Ramadan, performing Hajj to the house of Allah for those who are able, and all the other obligations that come in the pure sharee’ah.
Belief in Allah also include belief that he is the sole creator of the universe, the one regulates and governs affairs of his creation in accordance with His Knowledge and Power, as and how he wills. He is the master of the world and the hereafter and the entire universe. There is no creator and lord except him. None shares with him in any of these qualities.
“Allah is the creator of all things and he is the Wakeel (trustee, disposer of affairs, Gaurdian, etc) of all things” (39:62)
“indeed your lord is Allah who created the heavens and the earth in six days and then he rose above the throne. He covers the night with the day, seeking it rapidly, and he created the sun, the moon, the stars subjected to his command. Surely his is the creation and the commandment. Blessed be Allah, The Lord of the universe.” (7:54)
Belief in Allah include believing in his beautiful Names and loft Attributes. They have been mentioned in the Quran and by his messenger Muhammad (sallalahu alayhi wasallam). We believe in them without distorting the word or the meaning, denying the attribute, asking how it is, or resembling it to the creation. It is obligatory to leave them as they came without without going into the nature of the attribute or how they are, while believing in the meaning they carry.
“there is nothing like him, and he is the All-Hearer, the All-Seer” (42:11)
This the aqeedah of Ahlus Sunnah wal Jammah (those who follow the way of the prophet and unite upon it) from the companions of the messenger and those who follow them in goodness.
[Source: The Correct Islamic Aqeedah & that which opposes it, By Skaykh Abdul-Aziz Ibn Baz. Publisher: Dar us-Sunnah]