The aqidah of the Salaf with respect to Eemaan is summarized as:

• Eemaan is belief, speech and action, all of these enter into Eemaan and are from the reality of Eemaan. Thus, holding the belief that "Muhammad is the Messenger of Allaah" or "the Qur'an is the speech of Allaah" is Eemaan, and the feeling of love in the heart for Allaah, or hope or fear is Eemaan, and testifying "None has the right to be worshipped except Allaah alone" is Eemaan and saying "Subhaanallaah", or commanding someone "Fear Allaah and do not backbite" is Eemaan, and removing something harmful from the floor or performing prayer is Eemaan, and not stealing, or backbiting, and withholding one's limbs from falling into these actions is Eemaan. The outward actions of Eemaan are not merely the "fruits" and "consequences" of Eemaan as the Murji'at ul-Fuquhaa say, rather they are Eemaan itself.
• Eemaan increases with obedience and decreases with disobedience and it is not just a single indivisible entity that does not admit to increase, decrease - such that if some of it goes all of it goes (in the view of the Khawaarij and Mu'tazilah) or such that none of it goes no matter what actions one does (the Murji'ah) - as [in the erroneous view of both factions], Eemaan is just a single indivisible entity.
• It is permissible make an exception (al-istithnaa) for one's faith by saying "I am a believer, if Allaah wills", out of a) avoiding self-praise, and b) uncertainty about one's future actions and what one will die upon.
• kufr occurs by belief, speech and action and is not restricted to juhood and istihlaal only. Thus, a person becomes a disbeliever through a belief held (speech of the heart), a feeling, emotion harbored in the heart (pride, arrogance, hatred and so on [which is the heart's action]), a statement made by the tongue, or an action performed by the limbs.

And inshaa'Allaah these affairs will be documented and explained in future articles from the speech of the Salaf past and present. For now, on the subject of kufr (the opposite of eemaan) this is a reproduction of a ten-year old article (July 2000), with some additions:

Kufr (disbelief) Occurs By Beliefs, Statements and Actions

Imaan an-Nawawi said in "Kitaab ur-Riddah" (The Book of Apostasy), Rawdat ut-Taalibeen (7/284-283):

It is the cutting off of Islaam. This occurs sometimes by a statement which constitutes kufr, and sometimes by an act. And the actions which necessitate kufr (that expel from the religion) are those which are performed deliberately, and mocking the religion is clear [in this regard], such as prostrating to an idol or to the sun, or throwing the Qur-an into filth, and the magic which involves worshipping the sun and other such acts.

Ibn al-Qayyim said in Kitaab us-Salaat (pp.53-54):

And likewise, the branches of kufr (amongst them are) those which are statements and those which are actions. And just like a person can fall into disbelief (that expels from the religion) by wilfully saying a word constituting disbelief, and which is a branch of disbelief, then likewise, he can fall into disbelief (that expels from the religion) by doing an act of kufr, from amongst its branches that are actions, such as prostrating to an idol, belittling the mus-haf. So this is a principle.

Shaikh 'Abdur-Rahman bin Hasan said, Ad-Durar as-Sunniyah (p.214):

And likewise the Fuqahaa have mentioned concerning the ruling upon the apostate that a person can become a disbeliever by a statement (he utters) or an action he performs, even if he testifies that there is nothing which has the right to be worshipped except Allaah and Muhammad is His Messenger, prays, fasts and gives charity. He becomes an apostate by such statements and actions and whatever he uttered or did nullifies his actions, especially if he dies upon that. In that case the nullification of his actions is a matter of consensus.

Shaikh 'Abdur-Rahman bin Nasir as-Sa'dee said, Manhaj us-Saalikeen (p.112):

Chapter: The Ruling upon the Apostate. The apostate, murtad, is the one who exits from Islaam and enters into disbelief on account of an action, statement, a belief (i'tiqad) or doubt (shakk).

Shaikh Ibn Uthaimeen said in a cassette recording: Questions and Answers from Qatar (30th April, 2000):

And whoever Allaah and His Messenger declares to be a disbeliever, then he is a disbeliever, regardless of whether that is by way of an action (he committed), or a belief (that he held) or a statement that he uttered. And whoever Allaah and His Messenger have not declared to be a disbeliever, and he ascribes himself to Islaam, then he is a believer and it is not lawful for us to declare him a disbeliever.

Imaam al-Albani said in the cassette recording "Fitnah of Takfir" (with commentary by Shaykh Ibn Uthaymeen):

And amongst the actions are those on account of which a person actually disbelieves with the kufr of belief (i.e. apostatizes). This is because such actions show his disbelief with absolute certainty and decisiveness in the sense that when a person commits them, it is as if he is actually expressing his disbelief with his tongue, such as the one who kicks the Qur'an while knowing it is the Qur'an and intending to kick it, deliberately (i.e. with ilm and qasd for the action itself).

The Shaykh is speaking here about al-ilm (knowledge) and al-qasd (intent) from the angle of the requirements and barriers to takfir, such that a person willfully chooses and intends the action (which is kufr), and not that a person "intends" to commit kufr (through the action).

Shaikh Abdul-Azeez Bin Baz said as occurs in the magazine al-Furqan (no. 94).:

Sacrificing for other than Allaah and prostrating to other than Allaah is the kufr of action that expels from the religion. Similarly, if a person was to pray to other than Allaah or prostrate to other than Him, free is he of all imperfections, then he would disbelieve with the major kufr of action - and refuge is from Allaah. Likewise, if he was to abuse the religion, or abuse the Messenger (sallallaahu alaihi wasallam), or mock Allaah or the Messenger, then that is the major kufr of action in the view of all of Ahl us-Sunnah wal-Jamaa'ah.

Shaykh Saalih al-Fawzaan said in al-Muntaqaa (2/10):

There are actions which expel from the religion such as abandoning the prayer out of laziness, and magic, learning it and teaching it, and the one who spoke a word of kufr, wilfully (mukhtaaran, out of choice), and it is necessary that every action be accompanied with intent (qasd), thus the action of one who is forgetful, the one asleep, the small (child), the one who is insane, and the who is compelled is not considered due to the absence of intent (for the action).

The Shaykh is speaking here about al-qasd (intent) from the angle of the requirements and barriers to takfir, such that a person willfully chooses and intends the action (which is kufr), and not that a person "intends" to commit kufr (through the action).