What is faith according to Islam?

This is what the Qur'an says, in these verses and in many others:

'Praise be to Allah to Whom belong all things in the heavens and on earth. To Him be praise in the Hereafter: and He is All-Wise, All-Aware. He knows all that goes into the earth and all that comes out thereof; all that comes down from the sky and all that ascends thereto and He is the Most Merciful the Oft-Forgiving. The unbelievers say ‘Never to us' it will come upon you by Him Who knows the unseen, from Whom is not hidden the least little atom in the heavens or on earth, nor is there anything less than that, or greater, but is in the Record Perspicuous: That He may reward those who believe and work deeds of righteousness; for such is Forgiveness and a Sustenance most generous. But those who strive against Our Signs, to frustrate them, for such will be a chastisement of painful wrath. And those to whom knowledge has come see that the (Revelation) sent down to thee from thy Lord – that is the Truth, and that it guides to the Path of the Exalted (in Might), Worthy of all praise.' (34:1-6)

'O ye who believe! Give your response to Allah and His Messenger, when He calleth you to that which will give you life; and know that Allah cometh in between a man and his heart, and that it is He to Whom ye shall be gathered.' (8:24)

'Those whom Allah willeth to guide, He openeth their breast to Islam; those whom He willeth to leave straying, He maketh their breast close and constricted, as if they had to climb up to the skies; thus doth Allah lay abomination on those who refuse to believe. This is the Way of thy Lord, leading straight; We have detailed the Signs for those who receive admonition.' (6:125,126)

'Seest thou not that Allah doth know (all) that is in the heavens and on earth? There is not a secret consultation between three, but He is the fourth of them, nor between five but He is the sixth, nor between fewer nor more but He is with them wheresoever they be; in the end will He tell them what they did on the Day of Judgement, for Allah has full knowledge of all things. … O ye who believe! When ye are told to make room in the assemblies, make room: room will Allah provide for you. And when ye are told to rise up, rise up: Allah will raise up, to ranks those of you who believe and who have been granted knowledge. And Allah is well-aquainted with all ye do.' (58:7,11)

'Seest thou not those who turn to such as have the Wrath of Allah upon them? They are neither of you nor of them, and they swear to falsehood knowingly. Allah has prepared for them a severe Chastisement: evil indeed are their deeds. … Satan has got the better of them, so he has made them forget the remembrance of Allah. They are the Party of Satan. Truly, it is the Party of Satan that will lose. Those who oppose Allah and His Messenger will be among those most humiliated. Allah has decreed: ‘It is I and My messengers Who must prevail': for Allah is Strong, Mighty. Thou wilt not find any people who believe in Allah and the Last Day, loving those who oppose Allah and His Messenger, even though they were their fathers or their sons, or their brothers, or their kindred. For such He has written Faith in their hearts, and strenghtened them with a spirit from Himself. And He will admit them to Gardens beneath which Rivers flow, to dwell therein. Allah will be well pleased with them, and they with Him. They are the Party of Allah. Truly it is the Party of Allah that will achieve success.' (58:14,15,19-22)

'If Allah had so willed He could have made them a single people; but He admits whom He will to His Mercy; and the wrong-doers will have no protector nor helper. What! Have they taken protectors besides Him? But it is Allah, He is the Protector, and it is He Who gives life to the dead: it is He Who has power over all things. Whatever it be wherein ye differ, the decision thereof is with Allah: in Him I trust, and to Him I turn.' (42:8-10)

Islamic creed is acceptance of an almighty Creator introduced to man in a revealed traditional scripture, of which virtually everybody has at least some knowledge by name: The Qur’an. Faith lives inside the heart and mind and always starts with an inward travel to the soul, but also shows in behavior--or it doesn't. Every expression we make is consequence of our values: In the back of our mind we know, that this action is good or not. Believing people consider the reality of an almighty Creator knowing and judging this action and are more inclined to restrain themselves from actions that may be disapproved of by this supreme Authority. Faith, therefore, may be the label people give themselves, but it may also be its test in real life.

Please note, that ‘believing’ people have no stereotype name. There are two groups of people: Those who acknowledge the scripture and are willing to follow its suggestions and rules, and those who reject it. ‘Satan’, as the general name for evil and disobedience, is their main influence. This is another aspect: Why is disbelief evil, and will believing people feel no love for disbelieving people? This implies that ‘evil’ is not necessarily nor only a dogmatic refusal of a theoretical book, something which may be of no interest but to the individual reader him or herself. Evil can also be something that makes others suffer. Evil is real and harmful in practice, according to the rules of common sense, and it has many unexpected, not too pretty faces. However, why, after all, would a believer dislike the quiet neighbor just for his atheism? This shows another complicated side of belief and disbelief. Sometimes it is not expressed to others and does it simply live inside the mind. Allah knows it is there and will eventually sanction. People have a relationship with each other, but also with their God. The latter, however, is something other people have no authority over. Will Allah be your friend or foe? The individual person has a big say in this, whether he or she wants a friendly relationship with Allah or not. Allah is to finally judge this individual relationship with His creatures. A quiet harmless non-believer is still a non-believer with whom Allah will deal in the Hereafter. No disbeliever can say: I didn’t know. Works of art and even popular works mention the revealed scripture’s existence and summary. It is not possible to reject their texts solely on empirical evidence, as part of their contents are beyond our observational skills. Therefore religion judges this blind rejection as to be disapproved. So disbelief may be harmful  and destructive to the individual himself.

Faith is a two way stream, according to Islam. Those who accept the One God will be helped and inspired by Him, however, the One God also makes His own selection from among His creatures and opens or closes the heart to the Faith. To us, it is not always certain which of these prevails.