Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Shari'ah is the religious teachings that Allah has prescribed on His slaves. There is no difference between what Allah has prescribed and ordered in His Book and the Sunnah of His Prophet (Sallallahu Alaihi wa Sallam). This Sunnah includes the deeds, sayings and decisions of the Prophet (Sallallahu Alaihi wa Sallam). So, Shari'a includes believing in the Oneness of Allah, enjoining the good and forbidding the evil as well as performing the obligatory religious acts.
On the other hand, every Prophet (Alaihim al-Salaam) has a particular trend as far as some secondary rules of Shari'ah are concerned. But all of them are agreed on the fundamentals of the religion. Allah Says (interpretation of meaning): {… To each among you, We have prescribed a law and a clear way. …} [5: 48]. Allah also Says (interpretation of meaning): {He (Allah) has ordained for you the same religion (Islamic Monotheism) which He ordained for Nuh (Noah), and that which We have revealed to you (O Muhammad (Sallallahu Alaihi wa Sallam), and that which We ordained for Ibrahim (Abraham), Musa (Moses) and 'Isa (Jesus) saying you should establish religion (i.e. to do what it orders you to do practically), and make no divisions in it (religion) (i.e. various sects in religion). …} [42: 13].
Religion is the belief in the Oneness of Allah and His obedience. This cannot be done except through following His Prophets (Alaihim al-Salaam) and applying what is in His Books.
Ibn al-Arabi says: 'Among the things that are included in all the Shari'ahs of the Prophets (Alaihim al-Salaam) are believing in the Oneness of Allah, performing the prayers, giving the Zakah, fasting and performing Hajj as well as endeavoring to get nearer to Allah by performing good deeds such as being honest, fulfilling oaths, being loyal and trust worthy, having good ties with one's kindred. Shari'ah also includes forbidding disbelief, murder, Zina (fornication or adultery) and all harmful acts on people or animals. It also teaches people to stay away from all low and ignoble acts and from all that could demean their honour. All the above teachings are common among all the Shari'ahs of the different Prophets (Alaihim al-Salaam). Indeed all revelations mention the same teachings.
But some trends of Shari'ah might differ from one Prophet (Alaihi al-Salaam) to another according to the well-being of his nation or the other depending on what Allah Wishes. But the Islamic Shari'ah has abrogated all the Shari'as that came before it. Allah does not accept any religion or Shari'a after sending Prophet Muhammad (Sallallahu Alaihi wa Sallam) except Islam: Allah Says (interpretation of meaning): {Truly, the religion with Allah is Islam. …} [3: 19]. Allah also Says (interpretation of meaning): {And whoever seeks a religion other than Islam, it will never be accepted of him, and in the Hereafter he will be one of the losers.} [3: 85]. He also Says: {And We have not sent you (O Muhammad (Sallallahu Alaihi wa Sallam)) except as a giver of glad tidings and a warner to all mankind, …} [34: 28].
The Prophet Muhammad (Sallallahu Alaihi wa Sallam) also said: "By Him in whose hand is the life of Muhammad, he who amongst the community of Jews or Christians hears about me, but does not affirm his belief in that with which I have been sent and dies in this state (of disbelief), he shall be one of the denizens of Hell-Fire" .
Allah knows best.