Answer
Wa`alykum As-Salamu Warahmatullahi Wabarakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear brother in Islam, thanks a lot for your question, which shows your keenness on abiding by Allah’s laws and teachings. May Allah enlighten our hearts with the light of Islam!
Reflecting on the Qur’anic instructions and prohibitions, a Muslim will get convinced with the fact that it is his duty to keep himself away form all sins, minor and major. This emanates from the fact that it is the Muslim first priority to gain Allah’s pleasure by doing what He likes and steering clear of what He dislikes, irrespective of the gravity or the simplicity of the sin.
Coming to your question on which major sin stands in next to polytheism, in terms of its gravity, we may as well infer that from the order adopted by the Qur’an when dictating the duties of every Muslim. In the Qur’an immediately after the duty of worshipping Allah comes the duty of showing dutifulness to one’s parents: “Your Lord has decreed, that you worship none save Him, and (that you show) kindness to parents.” (Al-Israa’ 17:23)
It is well-known that Islam urges every Muslim to be dutiful to his parents, extending to them the kindest treatment possible. Failing to treat one’s parents kindly makes one guilty of disobedience to those parents as well as to Allah, and this may deprive one of a chance of being admitted to Paradise. So it goes without saying that disobedience to parents or mistreating them is the second major sin after polytheism.
This point is further clarified by Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, as follows:
The first commandment in Islam as revealed to all prophets is to worship Allah and the second is to treat one’s parents kindly. Accordingly, the first major sin is Shirk (association of partners with Allah), and the second major sin is mistreating one’s parents.
The Prophet (peace and blessings be on him) said: “Shall I tell you of the three most heinous sins?” He repeated this question three times and then said, “They are: to associate partners with Allah, to mistreat one’s parents, and to bear false testimony.” (Muslim)
In another tradition, the Prophet (peace and blessings be on him) said, “One who mistreats his parents cannot hope to enter Paradise.” (An-Nasa’i)
Also, he said, “The pleasure of Allah is in the pleasure of one’s parents and the wrath of Allah is in the wrath of one’s parents.” (At-Tirmidhi)
Now, it becomes clear that mistreatment of parents stands as the second major sin of Islam, after Shirk. Yet, a Muslim has to steer clear of all sins, major and minor, so as to gain Allah’s pleasure.
Allah Almighty knows best.
Quoted, with slight modifications, from: www.islam.ca