Praise be to Allah.
Islamic ‘aqeedah is not a theoretical and philosophical method; action is an essential part of this ‘aqeedah. Hence the Ahl al-Sunnah are agreed that eeman (faith) consists of both words and deeds, or speaking with the tongue, believing in the heart and acting with the physical faculties.
Whoever believes in Allaah as his Lord and God, will worship Him and obey Him by praying, giving zakaah and so on.
Whoever believes in the Last Day and its reckoning and reward or punishment, will be motivated thereby to do that which Allaah has commanded and abstain from that which He has forbidden.
Whoever believes that Muhammad (peace and blessings of Allaah be upon him) is the Messenger of Allaah will therefore obey him, follow his Sunnah and propagate his religion.
Thus the principles in which a person believes are translated into actions and words and efforts. The stronger that faith grows in his heart, the more its effects will be seen outwardly.
Hence the Prophet (peace and blessings of Allaah be upon him) said: “In the body there is a piece of flesh which, if it is sound, the entire body will be sound, and if it is corrupt, the entire body will be corrupt. It is the heart.” Narrated by al-Bukhaari, 52; Muslim, 1599.
Al-Hasan al-Basri (may Allaah have mercy on him) said: “Faith is not the matter of wishes or outward appearances, rather it is that which settles in the heart and is confirmed by one's actions.”
Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said: “If the heart is sound in terms of faith, knowledge and feelings, then this will inevitably lead to the soundness of physical actions, such as verbal statements and deeds. As the leading scholars of hadeeth said: Words and deeds, both inward and outward. What is outward must follow what is inward, so if what is inward is sound then what is outward will also be sound, and if it (the former) is corrupt, (the latter) will also be corrupt. Hence one of the Sahaabah said concerning the one who prayed carelessly: If his heart had been humble and focused, his body would have been humble and focused.
Majmoo’ al-Fataawa, 7/187
With regard to books which one may read about ‘aqeedah, there are many, the greatest of which are the Book of Allaah and the Sunnah of His Messenger (peace and blessings of Allaah be upon him), in which there is protection and salvation for those who adhere closely to them. The scholars were concerned with explaining and propagating sound ‘aqeedah, and they wrote innumerable books on this topic. Among the most famous of these books are the following:
Al-Sunnah by ‘Abd-Allaah ibn Ahmad ibn Hanbal
Al-Tawheed by Ibn Khuzaymah
Sharh Usool I’tiqaad Ahl al-Sunnah by al-Laalkaa’i
‘Aqeedat al-Salaf wa Ashaab al-Hadeeth by al-Sabooni
al-‘Aqeedah al-Waasitiyyah by Ibn Taymiyah [available in English translation]
al-‘Aqeedah al-Tahhaawiyyah wa Sharhuha by Ibn Abi’l-‘Izz al-Hanafi
Lawaami’ al-Anwaar al-Bahiyyah by al-Safaareeni
Ma’aarij al-Qubool by Haafiz Hakami
Al-Irshaad ila Saheeh al-I’tiqaad by Shaykh Saalih al-Fawzaan – this is an easy and useful book.
With regard to applying this ‘aqeedah in real life, this is done by learning it, propagating it, calling others to it, and refuting those who go against it with wisdom and beautiful preaching. This is how ‘aqeedah is spread and its effects made manifest so that all people may benefit from it.
The way in which ‘aqeedah may be learned is to learn it directly from those who believe in it and act accordingly. This is the soundest and best way for those who are able to do it. For those who are far away from scholars, they should refer to their commentaries, writings and tapes, as well as asking about confusing matters and anything they do not understand.
If a person follows some of the teachings of Islam and neglects others, by failing to do some of the obligatory duties or by committing some of the acts that are forbidden, this is a shortcoming in his faith and a weakness in his belief and his love for his Lord and His religion. This is undoubtedly a fault in his ‘aqeedah.
Hence one of the basic principles of Ahl al-Sunnah wa’l-Jamaa’ah is that faith increases by doing acts of worship and decreases by doing acts of sin. These shortcomings and faults may erase faith altogether, so that the person becomes an apostate from Islam, such as if he stops praying. See question no. 5208 and 2182.
With regard to sins that do not reach the level of kufr, such as withholding obligatory zakaah or looking at haraam things and so on, these are things which cause faith to decrease.
The Muslims need people to explain pure, correct ‘aqeedah to them, that is based on the Qur’aan and Sunnah according to the understanding of the righteous salaf. That is because of the widespread existence of ignorance, innovations (bid’ah), myths and deviant schools of thought.
Every Muslim must be sincere towards himself first of all, by learning correct aqeedah from trustworthy sources, then by propagating it and teaching it to the people, by means of lessons and lectures, books, publications and magazines, thus fulfilling the duty to convey the message as Allaah says (interpretation of the meaning):
“(And remember) when Allaah took a covenant from those who were given the Scripture (Jews and Christians) to make it (the news of the coming of Prophet Muhammad and the religious knowledge) known and clear to mankind, and not to hide it”
[Aal ‘Imraan 3:187]
“Let there arise out of you a group of people inviting to all that is good (Islam), enjoining Al-Ma‘roof (i.e. Islamic Monotheism and all that Islam orders one to do) and forbidding Al-Munkar (polytheism and disbelief and all that Islam has forbidden). And it is they who are the successful”
[Aal ‘Imraan 3:104]
“Say (O Muhammad): ‘This is my way; I invite unto Allaah (i.e. to the Oneness of Allaah — Islamic Monotheism) with sure knowledge, I and whosoever follows me (also must invite others to Allaah, i.e. to the Oneness of Allaah — Islamic Monotheism with sure knowledge). And Glorified and Exalted be Allaah (above all that they associate as partners with Him). And I am not of the Mushrikoon (polytheists, pagans, idolaters and disbelievers in the Oneness of Allaah; those who worship others along with Allaah or set up rivals or partners to Allaah)’”
And Allaah knows best.