For the benefits of our non-Muslim friends, let us introduce Islam to them. Islam is a monolithic religion, i.e the Muslims believe in the oneness of Allah (a.w), their God. Islam is Allah's religion, since the beginning of time,i.e. since the creation of the first human on earth, i.e. Adam. From Adam Allah (a.w) created the first woman, Hawa (i.e. the Christians calls Eve).
Allah (a.w) created the (seven) heavens and (seven layers of) the earth, 50,000 years before Allah (a.w) created man. Beginning with deportation of Adam and Hawa to earth, their decendants will inhabit the earth until the days of judgement. Adam was created from a mixture of earth (clay), thus resulting in the various shades of colors and characters of men from then on. Adam and Hawa were man's original ancestors.
When Adam was created, Satan, who was created from fire much earlier, refused to bow to Adam at Allah's command. Hence, Satan becomes the first creation of Allah to have transgressed Allah's command. When Adam was cajolled by Satan to plug and eat the fruit of the forbidden tree in Heaven, both Adam and Hawa were deported to earth, where their offsprings and decendents will live until the day judgement. Satan was also expelled. Satan vowed and promised to Allah to continue to influence decendants of Adam to disobey Allah, until the day of judgement. Hence Satan and his decendants continue to derail man from obeying Allah until now and will do so until the day of judgement, so that man will be punished together with him in Hell.
Allah said: 'I created not jinn and man but to serve (i.e. worship and obey) only me' [Verse 56, Surah az-Zariat]. That is the essence of Muslims cardinal law in the al-Quran. How to serve and obey Allah is laid down succintly in Verse 13, Surah an-Nisa': 'Whoever obeys Allah (a.w)(i.e. follow the al-Quran) and obeys the Prophet (Muhammad) (i.e. follow the as-Sunnah) will enter the heaven'. Otherwise, in Verse 14, Surah an-Nisa', Allah says: 'Whoever disobeys Allah (a.w)(i.e. reject or did not follow the prescriptions of the al-Quran) and whoever disobeys the Prophet (i.e. reject or did not follow the prescriptions of as-Sunnah), and whoever transgresses the limits (i.e. the don'ts) laid down by Allah (in the al-Quran and the don'ts of Rasulallah (in the as-Sunnah), will be thrown into the Hellfire.'. Man's final destination in life and hereafter is either the Heaven (for the obedients) and Hell (for disobedients and transgressors). In life and in serving and practising obedience to Allah, man is to be led by Rasulallah, who is the model 'par excellence' for man to emulate. He was to guide men to the right path to Heaven> Man is required to take and accept whatever Rasulallah (s.a.w) conveys to them and abstain from the don'ts stipulated by Rasulallah, i.e. Verse 7, Surah al-Haysr. This verse provides the basis for man's obedience to the commands of the Prophet in Islam, i.e. as-Sunnah,the second supreme guidance in life and in performing the obligations mandated by Allah (a.w).
The obligatory or mandatory acts that man has to undertake relates to the five daily prayers, the fasting during Ramadhan for a month, the giving out zakats (alms) and 'infaq fi sabilillah' (i.e. spending parts of our wealth in the path of Allah (a.w)), and the performance of the Hajj and Umrah, at least once in one's lifetimes. Once man follows the commands of both Allah (a.w) and the Prophet, he is free to enjoy the temporary life on earth, until he dies.
Hence, the Muslims have to obey the commands of Allah (a.w) and the (commands of the) Prophet in this world. Total obedience to Allah (a.w) (i.e. the al-Quran) and to the Prophet (i.e. the as-Sunnah), is the only way for man to be rewarded the Heaven, in the Hereafter. Otherwise, man will be punished in Hell for disobedience or transgression. In the even he transgresses or disobeys Allah's commands and the Prophets' guidance, he can repent (Taubat), and if Allah wills it, Allah pardons them and erase their sins. Only sins relating to 'syirik', i.e. belief in the multiplicity of Gods, that cannot be pardoned or forgiven! Hence Allah's condemnation of the Jews and the Chritians for assigning offsprings to Allah (a.w) as well as for assassinating His apostles and their refusal to acknowledge and accept Muhammad as Allah (a.w)'s final Apostle.
To be true Muslim, one has to abide by the prescriptions of the al-Quran and the as-Sunnah. As-Sunnah is the teachings of Prophet Muhammad (s.a.w). If one fails or if one resorts to or follows others's prescriptions or adopts and adapts the prescriptions of either the al-Quran and as-Sunnah, one is not accepted as a Muslim and Mukmin in Islam, although externally one seems to be a Muslim, but in reality one is not a Muslim. The prevailing controversies and polemics among Muslims are the results of their ignorance of al-Quran and as-Sunnah or their lack of understanding of the true teachings of the Prophet, or the after effects of misinterpretation of both al-Quran and as-Sunnah by some ulama' of the past!
Hence, the Muslims have to learn and understand, and accordingly, practise Islam, by the proper understanding of the al-Quran and the as-Sunnah. Hence the Prophet says that Muslims who abide by the al-Quran and as-Sunnah, will not deviate from the true path of Islam. By doing so, then and only then, they can be assured of success in their life in the Hereafter. Success means they are rewarded the bliss of the Heaven, failure means they will suffer punishments in Hell, whose fuel is men, i.e. non-believers, and idols, worshipped by man.
The Prophet reminded Muslims that in later days, after his death, the Muslims will be divided into 73 sects or groups, 72 groups in Hell and only one group will enter Heaven. The one group that enters Heaven is the one who strictly follows the Prophet's sunnah (i.e. his teachings) and the sunnah (i.e. the teachings) of the enlightened khalifahs succeeding him, namely Abu Bakar, Omar, Othman and Ali and they practise Islam in a congregation (i.e. Jema'ah). Unlike other religions, Islam is a congregational religion. That is to say, all mandatory obligations (eg. the five mandatory prayers) are to be performed in groups or congregations. Only additional obligations (eg. solat sunat) are complementary or recommended obligations, which must be performed invidually. Muslims who abide by the commands of Allah (a.w) (i.e. follows al-Quran) and the Prophet (i.e. as-Sunnah) will be rewarded the Heaven, others will be thrown into Hell, unless they repent and Allah forgives them or accepts their repentance (i.e. taubat).
Although Allah (a.w) commands that the Muslims obey 'ulil amri' (i.e. the hierarchical leaders) of Muslims' congregation, both the 'ulil amri and the congregation must resort to settlements of all disputes to the al-Quran and as-Sunnah, and that the 'ulil amri's decisions cannot supercede or override that of the al-Quran and as-Sunnah [Verse 59, Surah an-Nisa'].