Abu Hurairah relates that Allah’s Messenger ﷺ observed: The fornicator who fornicates is not a believer so long as he commits it, and no thief who steals is a believer as long as he commits theft, and no drunkard who drinks wine is a believer as long as he drinks it. Abdul Malik bin Abi Bakr narrated this on the authority of Abu Bakr bin Abdur Rahman bin Harith and then said: Abu Hurairah made this addition: No plunderer who plunders a valuable thing that attracts the attention of people is a believer so long as he commits this act.
(Sahih Muslim)
Iman is intense God-consciousness. A true believer is he who always feels that he is living in the presence of an All-Knowing, Omnipresent and All-Powerful Lord, and nothing is hidden from His ever-watchful eye. Such heinous crimes as fornication, plundering, theft and drinking are the very negation of faith. A man who commits such a crime is bereft of God’s consciousness even though for a short time, because if he were aware that he is being watched by the Lord and he shall be punished in the Hereafter for such a misdeed, he would never violate His commands and commit such an evil.
This hadith is indicative of the fact that Iman is not something static. It increases and decreases and seldom remains constant. But it should be made clear that Iman in this hadith refers to the state of God’s consciousness, inner piety and a sense of accountability of one’s deeds.