Indeed Eman according to Ahlus Sunnah wal Jamaa’ah is the Iqrar [strong affirmation] with the heart, speech with the tongue, and actions with the body parts. So, it includes these three matters;
affirmation with the heart,
speech with the tongue and
actions with the body parts.
Since this is the case, then surely it will increase and decrease, and this is because the affirmation with the heart varies from person to person. Hearing about something is not like seeing it yourself, and one person informing you of something is not the same as two people informing you, and so on.
This is why Ibraheem - عليه الصلاة و السلام - said:
And (remember) when Ibraheem said: “My lord show me how you give life to the dead?” He (Allah) said: “Do you not believe?” He Ibraheem said: "Yes, but so that my heart may be stronger and more tranquil in Eman.”
Al-Baqarah: 260 So indeed Eman increases in regards to the affirmation of the heart, its tranquility and its serenity. One notices this in himself, when one sits in a gathering of the remembrance of Allah; a sermon is being delivered mentioning Jannah and the Fire, his Eman can increase to the point where he feels as if he can see them with his eyes. Likewise, at times when one is heedless and leaves such a gathering, this would decrease the Yaqeen (surety and strength of Eman) in his heart.
Eman increases with regards to speech as well. For indeed, the one who mentions Allah ten times is not like the one who mentions Allah a hundred times, the second one increases by a lot.
Likewise the one who practices ‘Ibadah (worship) in the most perfect manner, his Eman will increase more than the one who practices it in a deficient manner. The same applies to actions. If a person performs more acts of worship with his limbs than another, then the former would increase Eman more than the latter, who would have less.
Indeed the Book and the Sunnah has confirmed the increase and decrease of Eman, Allah the Glorified said:
And We have not made the keepers of the Fire except angels. And We have not made their number except as a trial for those who disbelieve - that those who were given the Scripture will be convinced and those who have believed will increase in faith
Allah the Majestic also says:
And whenever a Surah is revealed, there are among them [i.e., the hypocrites] those who say, " Whom from amongst you has increased in faith by it?" As for those who believed, it has increased them in faith, while they rejoice.
In the authentic Hadeeth the Messenger of Allah صلى الله عليه و سلم said:
I have never seen anyone with a deficiency of ‘aql (An Nawawee said: meaning they have weak memories) and Deen (religion –meaning they do not pray or fast during their menses) that are more overpowering to men than one of you.
فالإيمان يزيد من حيث الإقرار إقرار القلب وطمأنينته وسكونه، والإنسان يجد ذلك من نفسه، فعندما يحضر مجلس ذكر، فيه موعظة وذكر للجنة والنار، يزداد إيماناً حتى كأنه يشاهد ذلك رأي عين، وعندما تكون الغفلة، ويقوم من هذا المجلس، يخف هذا اليقين في قلبه.
كذلك يزداد الإيمان من حيث القول، فإن من ذكر الله عز وجل عشر مرات، ليس كمن ذكر الله مائة مرة، فالثاني أزيد بكثير.
وكذلك أيضاً من أتى بالعبادة على وجه كامل، يكون إيمانه أزيد ممن أتى بها على وجه ناقص، وكذلك العمل، فإن الإنسان إذا عمل عملاً بجوارحه أكثر من الآخر، صار الثاني أزيد إيماناً من الناقص، وقد جاء ذلك في الكتاب والسنة، أعني إثبات الزيادة والنقصان جاء في الكتاب والسنة، قال الله تبارك وتعالى: