On the day of Feast of Ghadir Khum, people came in groups to renew their pledge of allegiance with the Prophet (PBUH) and swear allegiance with Imam Ali and also to congratulate Imam Ali on his appointment as successor to the holy Prophet.
Gabriel descended saying "I swear by God I have not seen any scene as magnificent as this. He confirmed the situation of his cousin! Paid him a pledge of allegiance that only the unbelievers of God and His Prophet may break and cursed those who break it!"
In addition to declaring Imam Ali as his successor, the Prophet planned a line of succession on the day of Ghadir, which disappointed the enemies until the Last Day. The religious mission was completed on this day: "This day have I perfected for you your religion and completed my favor on you and chosen for you Islam as religion." (5:3) God was pleased with the Prophetic mission. Religion would become complete with Prophet's declaration of the whole religion in that gathering, including "Imamate and Velayat".
Islam was completed because of confirmation of a succession line. On that occasion, not only Ali was declared as the successor of the Prophet but also the glorious line of "Imamate and Velayat" was introduced to the world. Ali became the Imam of all believers. God chose Islam to remain until the eternity because succession of Imam Ali disappointed the enemies who thought Islam may fade by the departure of the Holy Prophet. The Prophet (PBUH) declared the line of Imamate so that the religion of God may remain for times to come.
He introduced his line of succession from the children of Ali to be followed for salvation.
The Prophet (PBUH) declared repeatedly "loving Ali and his children means loving me, and, hostility towards them is hostility towards me".
source : www.tebyan.net
