Praise be to Allah.
According to Ahl al-Sunnah wa’l-Jamaa’ah, eemaan (faith) is “belief in the heart, words on the tongue and actions of the body.” This includes three things:
1.Belief in the heart
2.Words on the tongue
3.Actions of the body
As this is the case, then it will increase and decrease. That is because the degree of belief in the heart differs from one person to another. Believing in what someone tells you is not the same as believing something that you see with your own eyes. Believing in what one person tells you is not the same as believing what two people tell you, and so on. Hence Ibraaheem (peace be upon him) said:
“ ‘My Lord! Show me how You give life to the dead.’ He (Allaah) said: ‘Do you not believe?’ He [Ibraaheem (Abraham)] said: ‘Yes (I believe), but to be stronger in Faith’” [al-Baqarah 2:260]
Eemaan may increase with regard to belief, contentment and tranquillity in the heart. A person may experience that in himself when he attends gatherings of dhikr where he hears a lesson. Remembering Paradise and Hell increases eemaan to the point where it as if you can see them with your own eyes. But when a person is negligent and leaves those gatherings, this certain faith is reduced in his heart.
Eemaan in the sense of words may increase similarly. Whoever remembers Allaah a few times is not like one who remembers Him one hundred times. The latter is far better in eemaan.
And the one who does an act of worship in a perfect manner will have more faith than one who does it in an imperfect manner.
The same applies to righteous deeds. If a person does good deeds physically more than another, the one who does more will have more eemaan than the one who does less. This is stated in the Qur’aan and Sunnah – I mean the fact that eemaan may increase and decrease. Allaah says (interpretation of the meaning):
“And We have fixed their number (19) only as a trial for the disbelievers, in order that the people of the Scripture (Jews and Christians) may arrive at a certainty [that this Qur’aan is the truth as it agrees with their Books regarding the number (19) which is written in the Tawraat (Torah) and the Injeel (Gospel)] and that the believers may increase in Faith [al-Muddaththir 74:31]
“And whenever there comes down a Soorah, some of them (hypocrites) say: “Which of you has had his Faith increased by it?” As for those who believe, it has increased their Faith, and they rejoice.
But as for those in whose hearts is a disease (of doubt, disbelief and hypocrisy), it will add suspicion and doubt to their suspicion, disbelief and doubt; and they die while they are disbelievers [al-Tawbah 9:124-125]
According to a saheeh hadeeth, the Prophet (peace and blessings of Allaah be upon him) said: “I have never seen any women lacking in reason and faith who have a more negative impact on the mind of a serious man than you.”
Thus eemaan increases and decreases.