Short answer
Sunni scholars agree that a believer's faith grows stronger with obedience and weaker with sin. Imam Abu Hanifa and the Maturidis say the core of iman, belief in what must be believed, does not itself go up or down; what grows is its strength and light. Most Ash'aris and Atharis say iman itself increases and decreases.
In the Hanafi-Maturidi creed, iman is affirmation in the heart and declaration with the tongue. Deeds are not part of its definition, though they are required and they strengthen it. Its core cannot shrink, because doubting any part of it would be disbelief, but its certainty, light and fruits grow and weaken.
The four schools agree on this.
Explanation
The Quran says the believers' faith increases when Allah's verses are recited to them (8:2), and that Allah sent tranquillity into their hearts so they would increase in faith along with their faith (48:4). Hadith describe faith as having over seventy branches, and changing evil only in the heart as the weakest of faith. Everyone agrees on what these texts show: believers are not all at the same level.
The difference is about what exactly grows. For the majority of the Ash'aris and the Atharis, deeds are part of iman, so iman itself rises with obedience and falls with sin. For Imam Abu Hanifa and the Maturidis, iman is the heart's firm acceptance of all that the Prophet ﷺ brought, with the tongue confirming it. That acceptance is either complete or it is not faith, so its core does not shrink. They read the verses as an increase in what was believed while revelation was still coming, and in certainty and light.
Many scholars, among them Fakhr al-Din al-Razi, said the difference is largely one of words. Both sides agree that deeds are necessary, that sins weaken a believer, and that a Muslim who sins is not a disbeliever. Both agree that the faith of a prophet is far above ours in strength and light.
The practical lesson is shared: feed your faith. Prayer, Quran, dhikr, good company and serving others strengthen it, and sins dull it. When you feel your faith is low, that feeling is itself a sign of faith; turn back to Allah. See also the six articles of faith.
The evidence
Quran, Surah Al-Anfaal 8:2
إِنَّمَا ٱلْمُؤْمِنُونَ ٱلَّذِينَ إِذَا ذُكِرَ ٱللَّهُ وَجِلَتْ قُلُوبُهُمْ وَإِذَا تُلِيَتْ عَلَيْهِمْ ءَايَٰتُهُۥ زَادَتْهُمْ إِيمَٰنًۭا وَعَلَىٰ رَبِّهِمْ يَتَوَكَّلُونَ ﴿٢﴾
The believers are only those who, when Allah is mentioned, their hearts become fearful, and when His verses are recited to them, it increases them in faith; and upon their Lord they rely -
Faith grows when the Quran is heard.
Quran, Surah Al-Fath 48:4
هُوَ ٱلَّذِىٓ أَنزَلَ ٱلسَّكِينَةَ فِى قُلُوبِ ٱلْمُؤْمِنِينَ لِيَزْدَادُوٓا۟ إِيمَٰنًۭا مَّعَ إِيمَٰنِهِمْ ۗ وَلِلَّهِ جُنُودُ ٱلسَّمَٰوَٰتِ وَٱلْأَرْضِ ۚ وَكَانَ ٱللَّهُ عَلِيمًا حَكِيمًۭا ﴿٤﴾
It is He who sent down tranquillity into the hearts of the believers that they would increase in faith along with their [present] faith. And to Allah belong the soldiers of the heavens and the earth, and ever is Allah Knowing and Wise.
Speaks of faith increasing.
Quran, Surah Al-Hujuraat 49:14
۞ قَالَتِ ٱلْأَعْرَابُ ءَامَنَّا ۖ قُل لَّمْ تُؤْمِنُوا۟ وَلَٰكِن قُولُوٓا۟ أَسْلَمْنَا وَلَمَّا يَدْخُلِ ٱلْإِيمَٰنُ فِى قُلُوبِكُمْ ۖ وَإِن تُطِيعُوا۟ ٱللَّهَ وَرَسُولَهُۥ لَا يَلِتْكُم مِّنْ أَعْمَٰلِكُمْ شَيْـًٔا ۚ إِنَّ ٱللَّهَ غَفُورٌۭ رَّحِيمٌ ﴿١٤﴾
The bedouins say, "We have believed." Say, "You have not [yet] believed; but say [instead], 'We have submitted,' for faith has not yet entered your hearts. And if you obey Allah and His Messenger, He will not deprive you from your deeds of anything. Indeed, Allah is Forgiving and Merciful."
Shows the difference between outward submission and faith in the heart.
Sahih Muslim 35
Sahih... الإِيمَانُ بِضْعٌ وَسَبْعُونَ ...
"Faith has over seventy branches."
Faith has many branches and levels.
Sahih Muslim 49
Sahih... وَذَلِكَ أَضْعَفُ الإِيمَانِ ...
"And that is the weakest of faith."
Faith has degrees of strength.
Where the ruling is recorded
- al-Fiqh al-Akbar (Imam Abu Hanifa), Section on iman. Defines iman as affirmation and declaration, and says it does not increase or decrease in what is believed.
- al-Aqida al-Tahawiyya, Section on iman. States that iman is one, its people are equal in its root, and they differ in fear of Allah and deeds.
- Sharh al-Aqa'id al-Nasafiyya (al-Taftazani), Section on whether iman increases and decreases. Sets out both positions and the view that the difference is largely verbal.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.