Conclusion of Prophethod and Accomplishment of Religion
As the Holy Quran explicitly states, the Prophet of Islam is the last prophet and there will be no prophets after him.
ما كانَ مُحَمَّدٌ أَبا أَحَدٍ مِنْ رِجالِكُمْ وَلكِنْ رَسُولَ اللَّهِ وَخاتَمَ النَّبِيِّينَ وَكانَ اللَّهُ بِكُلِّ شَىءٍ عَلِيما
Muhammad is not the father of any of your men, but (he is) the Apostle of God, and the Seal of the Prophets: and God has full knowledge of all things.
The end of prophet hood is a fundamental idea in Islam and the Islamic Ummah has consensus over it. Therefore, Islam is a religion forever and suffices human need for a religion in all times and all places. Moreover, since Prophet Muhammad (s) is the last prophet, revelation has ended after him. This is explicitly stated in Nahjul Balaghah of Imam Ali (`a) and some traditions.
Amiral Mu’minin (`a) stated, “The Exalted Allah sent Prophet Muhammad (s) at a time when there were no prophets and the people were in controversy. Therefore, He sent the Prophet (s) after other prophets and the revelation ended after him.” 
As a result, at the time of Prophet Muhammad’s (s) demise all religious commandments and rules should have been presented to the people and the religion been completed, as is explicitly stated in the Holy Quran.
The Holy Quran says,
الْيَوْمَ يَئِسَ الَّذِينَ كَفَرُوا مِنْ دِينِكُمْ فَلا تَخْشَوْهُمْ وَاخْشَوْنِ اليَوْمَ أَكْمَلْتُ لَكُمْ دِينَكُمْ وَأَتْمَمْتُ عَلَيْكُمْ نِعْمَتِى وَرَضِيتُ لَكُمُ الإِسْلامَ دِيناً
This day have I perfected your religion for you, completed My favor upon you, and have chosen for you Islam as your religion.
There is controversy among Quran interpreters about the important day mentioned in this verse, in which Islam is completed and stabilized and the infidels got disappointed of nullifying it. After precise detailed discussions and reviewing some traditions, the late Allamiyi Tabataba’i concludes that the important day mentioned in the Quranic verse is eighteenth of Zil Hajjah (the last lunar month) and during Prophet Muhammad’s Hajjatul Wida’ (the last hajj pilgrimage). As mentioned in traditions and history books, the Messenger of Allah (s), who was returning from his last hajj pilgrimage, gathered the pilgrims in Ghadir land on this day. After performing a sermon, he formally appointed Ali Ibn Abi Talib (`a) to the Imamate position and during a long tradition he said, “Anyone whose Imam is I, so now Ali (`a) is his Imam. O Lord! Love those who love him and dislike those who dislike him!”
The Messenger of Allah (s) did two things in that historical day. First, he introduced the Holy Quran and his Household (the Ahlul-Bayt) as two valid scientific sources, saying, “Anyone who follows these two sources will not go astray.” Second, he introduced Imam Ali (`a) as his first successor and Imam from the Ahlul-Bayt (`a). Since the Prophet of Islam (s) had chosen Ali (`a) as the guardian of prophet hood sciences during his prophecy and attempted well in training him, Islam was completed by appointing him as the Imam of the ummah; a trustworthy guardian and executer of religious sciences. In such conditions, the mentioned verse was revealed to the Messenger of Allah (s).
How the ‘completion of religion’ by the Almighty Allah is justified, except that the Prophet (s) taught all religious commandments and rules part by part or wholly to Imam Ali (`a) during his lifetime and to other Imams (`a) via Imam Ali (`a). It should also be remembered that the rules and commandments mentioned in the Holy Quran or left from Prophet Muhammad (s) do not satisfy the needs of people in that age and later eras. That is why the Sunni jurisprudents were forced to use qiyas (analogy), istihsan (using Imams’ speech), or other principles for deducting religious commandments.
 Surah Al-Ahzab (33): 40.
 Nahjul Balaghah, Sermon 133.
 Surah Al-Ma’idah (5): 3.
 Al-Mizan Exegesis, Vol 5,s pp. 214-277.