The sermon of Ghadeer was the greatest announcement in the history of the Prophet (SAWA) in which he declared the order of Allah (SWT) for the future of Islam and its divine leadership after the Prophet. The Prophet (SAWA) asked tens of thousands of Muslims who were returning with him from Hajj: Am I not more entitled on you than yourselves on yourselves? All said: Yes you are. He raised the hand of Ali (AS) and said: For whom I am the master, Ali is his master. The Prophet (SAWA) then asked all companions who were present to give allegiance to Ali as the divine leader after the Prophet.
After Ghadeer announcement, Allah (SWT) revealed the Quranic verse: Today I Have perfected for you your religion, and completed on you My bounties and approved for you Islam as your religion (Sura 5, Verse 3).
This is a clear Quranic evidence that with out Ghadeer announcement, Religion was not perfected, the bounties from Allah were not completed and Islam with out the divine leadership was not approved by Allah.
Wassalam.