Prophet Muhammad (sal Allahu alaihi wa sallam) said: "O Aishah, abstain from the sins which are looked upon as trivial, for they too will be inquired about by Allah." [Musnad Ahmad, Nasa'i]
Sins are termed as major or minor essentially because of their gravity in terms of their effects on individuals and society. Thus, some sins are considered most heinous and incur divine wrath. Among such sins are: shirk (associating partners with Allah), displeasing one’s parents, murder, bearing false testimony, adultery, fornication, practicing black magic, devouring riba (interest), stealing from an orphan’s wealth, etc.
Minor sins, on the other hand, are minor in so far as the punishment for a single act of one of these is not as severe as for a major sin. Nevertheless, all sins require tawbah (sincere repentance). Sincere repentance involves four steps: The first is for a person to be deeply remorseful of the sin; secondly, to refrain from it and all of the circumstances that led to it; thirdly, to make a firm resolution never to do it again; and finally, to follow it up by whatever good deeds one can possibly do. Allah (subhana wa ta’ala) says in the Quran, “Verily good deeds wipe out bad deeds.” [Hud: 114] The Prophet (sal Allahu alaihi wa sallam) further explained this by saying: “Allah does not wipe out bad deeds through bad deeds; rather He blots them out through good deeds.”
To repent, one must quit sinful behaviour cold turkey and also cut oneself off totally and entirely from all associations or circumstances that led him/her to such sins. He/she cannot hope to change for the better without such serious measures. If, instead, one persists in the sin, he/she will end up losing the purity of their heart, their moral conscience.
The Prophet (sal Allahu alaihi wa sallam) warned us against the pervasive effects of sins on our souls. He said that when a person commits a sin, it falls like a black dot on the clean slate of his heart. If he repents, the dot is erased. If, however, he persists in the same and continues to sin, the dot continues to expand until it covers his entire heart, in which case his spiritual soul becomes completely sealed off. As a result, he fails to recognize good as good and evil as evil. The Prophet (sal Allahu alaihi wa sallam) said that this is what Allah (subhana wa ta'ala) has warned us against in the Quran: “Nay, their hearts have been sealed by the sins they have accumulated.” [Al-Mutaffifeen: 14]
Abdullah bin Mas'ud (radi Allahu anhu) reported Rasul Allah (sal Allahu alaihi wa sallam) as saying: "Beware of minor sins, for they will gather together on man so much so that they will kill him. [Musnad Ahmad]