The Messenger of Allah (s) says:
“Shall I inform you of the greatest sins? Associating others with Allah, being undutiful to parents, and false words.”
Sin and crime are both evil for people. Associating other partners with Allah, being undutiful to parents, and uttering false words are all mentioned by the Holy Prophet (s) in this hadith as being among the greatest sins. Others include theft, murder, deceit, gambling, sorcery, and harming others...etc.
All cause as much harm to their doers as they harm other people. Indeed, Allah promises the wrongdoers who commit these offences severe punishment, i.e. hell, but some of these sins are worse than others. This is because they are more harmful and endanger the lives of people, and also because they cause sins.
The most abominable of these atrocious sins are the following:
Associating other partners with Allah is to believe in the existence of a god other than Allah, or to say that Allah has sons as the Jews and the Christians allege, and Whose beginning is like that of other creatures. Such beliefs are considered among the greatest sins because they cause people to deviate from the straight path, and it is telling lies about Allah.
And he whose belief is corrupt and lies against Allah, does not respect people, nor guards the welfare of humanity nor forbids crimes and wrongdoing. That is why polytheism is among the greatest sins.
A Muslim's heart is filled with love and kindness towards parents. Both have favoured for us and are the origin of our existence in this life. Our education, instruction and care are due to them. No one forgets his father's favour, who strives to work and supports his family. And no one forgets his mother's favour, bearing us in her womb while her life blood nourishes us, and watching over us many a night sleepless for our sake, keeping us, rearing us and educating us.
Parents are the source of love and benevolence for each individual in society, and whoever wrongs their parents and does not carry out his duties towards them is surely an unrighteous person, having no respect and being ungrateful to those who bestowed such benevolence upon us. Such a person will become habitual in doing evil against others and extorting from them their rights.
The Holy Messenger (s) instructs us that being undutiful towards our parents is among the greatest sins, and Allah is wrathful with those who wrong their parents and deny them respect.
Speaking false words is denying what is true. Indeed, the Holy Messenger (s) says: It is considered among the greatest sins because the liar causes harm both to people and to himself.
Lying is the result of doing various evil deeds, like a thief stealing property and denying it belongs to others. When asked why he has stolen another's property; he will claim that he did not take it. Or there is the oppressor who denies people their basic rights and when he is asked why he behaves in such a bad way, he lies about it.
Lying is the route to destruction of all moral behaviour in contrast to being really truthful and confessing misdeeds to ensure they are not committed again.
It is said one lie leads to another, and the Messenger Muhammad (s) says: Surely, lying is among the greatest sins due to its being the cause for crimes, and when man gives it up and is truthful, he will be far from such crimes.