We are grateful to God that He has bestowed upon us the blessing of the Wilayah (guardianship) of Amir al-Mu'minin (the Commander of the Faithful) and the Imams (peace be upon them):
"الْحَمْدُ لِلَّهِ الَّذِي جَعَلَنَا مِنَ الْمُتَمَسِّكِينَ بِوِلايَةِ أَمِيرِالْمُؤْمِنِينَ وَ الأَئِمَّةِ عليهم السلام"
(Absolute-Praise be to God who has made us of those who hold fast to the Wilayah of Amir al-Mu'minin and the Imams, peace be upon them). We are grateful to God that He has bestowed upon us this blessing, which is the Divine manifestation.
The Wilayah of the Ahl al-Bayt (peace be upon them) is the completion of the mission of the noble Messenger of God (may God's blessings and peace be upon him and his family), for if there is no Wilayah, no Imamate, and no successorship, neither the Qur'an would remain in the society, nor the luminous statements of the Prophet. The Exalted God considered the Wilayah as the completion of the Prophethood, and said that if there is no Wilayah, the Prophethood will be fruitless: ﴿يا أَيُّهَا الرَّسُولُ بَلِّغْ ما أُنْزِلَ إِلَيْكَ﴾ (O Messenger! Convey what has been revealed to you) meaning convey the course of Wilayah, ﴿وَ إِنْ لَمْ تَفْعَلْ فَما بَلَّغْتَ رِسالَتَهُ﴾ (and if you do not do so, then you have not conveyed His message). If you do not convey this course to the society, the essence of your mission will be incomplete. When it is said that if you did not say, you did not do anything, it does not mean that if you did not say about the Wilayah, you neglected the Wilayah; this would be the unity of the antecedent and the consequent, which is not beneficial!
Since his mission is related to the essence of religion, the conclusion is that if you do not convey the Wilayah of Ali ibn Abi Talib (may God's blessings and peace be upon him), you have not conveyed the essence of the mission.
If there is no Wilayah, the divine mission has not been accomplished!