What is the definition of faith according to the Sunnis and the Community, and does it increase and decrease?

According to the Ahl al-Sunnah wal-Jama'ah, faith is affirmation by the heart, pronouncement by the tongue, and action by the limbs, and therefore it increases and decreases. The increase in faith comes from the heart's affirmation and tranquility, from frequent remembrance of Allah, from performing acts of worship completely, and from an abundance of actions by the limbs. Scriptural texts have indicated the increase and decrease of faith, such as the Almighty's saying: "And We did not make their number except as a trial for those who disbelieve - that those who were given the Scripture would be convinced and those who have believed would increase in faith" (Al-Muddaththir/31), and His saying: "And whenever a surah is revealed, some of them say, 'Which of you has this increased in faith?' As for those who believed, it increased them in faith, and they rejoice" (At-Tawbah/124).