We know that there are major and minor sins. Allah, Exalted be He, says: {Those who avoid major sins and shameful acts, except for minor lapses.} [Surat an-Najm: 32] He Almighty also says: {If you avoid the major sins which are forbidden to you, We will absolve your [minor] sins and admit you to a noble entrance.} [Surat an-Nisā’: 31]
‘Abdullāh ibn Mas‘ūd (may Allah be pleased with him) reported: A man said: "O Messenger of Allah, which sin is the gravest in the sight of Allah?" He said: "To ascribe an equal to Allah while He created you." He said: "Then what?" He said: "To kill your child out of fear that he will eat along with you." He said: "Then what?" He said: "To commit adultery with the wife of your neighbor." Then, in confirmation of this, Allah Almighty revealed the verse that says: {And those who do not invoke besides Allah another god, and who do not kill a soul whom Allah has forbidden, except in the course of justice, and who do not commit adultery.} [Surat al-Furqān: 68] [Narrated by Al-Bukhāri (6001) and Muslim (86)]
We know that Imān increases and decreases. Allah, Glorified be He, says: {Whenever a Chapter is sent down, some of those [hypocrites] say: "Which of you has this increased in faith?" As for those who believe, it does increase them in faith and they rejoice.} [Surat at-Tawbah: 124] Good deeds increase and establish faith, whereas sins decrease and weaken it. A sin, below disbelief and Shirk, contradicts the perfect state of faith. A perpetrator of a major sin should not be stripped of faith altogether and does not deserve to be described as an absolute believer, for he has done something that undermines his faith. Hence, the Prophet (may Allah's peace and blessings be upon him) said: "An adulterer is not a believer at the time he is committing adultery; a drinker of alcohol is not a believer at the time he is drinking alcohol; a thief is not a believer at the time he is stealing; and a robber is not a believer at the time he is robbing and people are looking on." [Narrated by Al-Bukhāri (2475) and Muslim (57)] In another Hadīth, the Prophet (may Allah's peace and blessings be upon him) said: "Whoever amongst you sees an evil should change it with his hand; if he is unable to do so, then with his tongue; and if he is unable to do so, then with his heart; and that is the weakest form of faith." [Narrated by Muslim (49)] About what will happen on the Day of Judgment, the Prophet (may Allah's peace and blessings be upon him) said: "Allah Almighty will then say: 'Go, and whomever you find in his heart a dinar's worth of faith, bring him out.'" [Narrated by Al-Bukhāri (7493) and Muslim (183)] These Hadīths indicate that sins weaken faith.
A perpetrator of a major sin is threatened with punishment for his sin, unless he repents from it, faces the legal penalty, if his sin is one that entails such a penalty, or Allah Almighty forgives him due to any of the numerous reasons for forgiveness. Our Lord says: {Allah does not forgive associating partners with Him, but forgives anything less than that for whom He wills.} [Surat an-Nisā’: 48]