How do we define faith? What is faith of one person in islam and in general?
Iman (faith\belief) literally means accepting something to be true.
In Islamic nomenclature it is defined as accepting everything, which is an essential part of the religion sent by Allah upon Muhammad ﷺ, to be true. By heart, by tongue and by actions*. And it is the antonym of kufr.
الايمان: ... تصديق رسول الله صلى الله عليه وسلم فيما جاء به عن ربه تبارك وتعالى
الإيمان: ... هو تصديق سيدنا محمد - صلى الله عليه وسلم - في جميع ما جاء به عن الله تعالى مما علم مجيئه ضرورة
Some of the constituents of Iman are documented in ahadith such as the following:
وقال " هل تدرون ما الإيمان بالله " . قالوا الله ورسوله أعلم . قال " شهادة أن لا إله إلا الله وأن محمدا رسول الله وإقام الصلاة وإيتاء الزكاة وصوم رمضان وأن تؤدوا خمسا من المغنم
He asked them, "Do you know what is meant by belief in Allah?
They said, "Allah and His Apostle know best."
Thereupon the Prophet (ﷺ) said, "To testify that none has the right to be worshipped but Allah and that Muhammad is His Apostle, to offer prayers perfectly, to pay Zakat, to observe fasts during the month of Ramadan, (and) to pay Al-Khumus (one fifth of the booty to be given in Allah's cause)."
أن جبريل، عليه السلام قال للنبي صلى الله عليه وسلم ما الإيمان قال أن تؤمن بالله وملائكته وكتبه ورسله واليوم الآخر وبالقدر خيره وشره
Jibreel عليه السلام said to the Prophet ﷺ: What is faith?
He said:“To believe in Allah, His Angels, His Books, His Messengers, the Last Day and the divine decree, both good and bad.”
Faith(emaan) is belief in the heart, expression upon the tongue & action upon the limbs. It increases with good deeds and decreases with bad deeds. Hence the level of faith(emaan) differs from person to person.
From below evidence we can conclude that believing in the heart will not suffice. And likewise faith(emaan) upon the tongue also will not suffice, because this is the faith(emaan) of the hypocrites,
They (the hypocrites) say upon their tongues that which is not in their hearts (Sooratul-Fath (48), aayah 11)
Below evidence proves faith(emaan) increases and decreases with actions. And it increases through obedience and decreases through disobedience.
And when His ayaahs are recited to them, it increases them in eemaan (Sooratul-Anfaal (8), aayah 2)
The believers are only those who, when Allâh is mentioned, feel a fear in their hearts and when His Verses (this Qur'ân) are recited unto them, they (i.e. the Verses) increase their Faith; and they put their trust in their Lord (Alone) [Surah Anfaal 8:2]
Abu Huraira reported: The Messenger of Allah(ﷺ), said, “Faith has sixty to seventy branches, the best of which is to declare there is no God but Allah, the least of which is to remove something harmful from the road, and modesty is a branch of faith.” Source: Ṣaḥīḥ al-Bukhārī 9, Ṣaḥīḥ Muslim 35