The Holy Qur’an states that the aim and purpose of the Islamic state is to establish the virtues that God wants in society. Thus there are two dynamics; the Theological and the Political:
Shia Muslims establish that Islam has always appointed leadership and that the Leadership of Imam Ali was appointed from God. Sunni Muslims believe that Leadership after the last Prophet was not appointed and the Leadership should be selected among the people.
Sunni Muslims believe that there is no need for appointed Leadership from God after the last Prophet . Sunni Muslims believe the Holy Quran and the Hadith (Traditions of the Prophet) is all we need from God to understand the virtues that God wants in society. Shia Muslims believe that God has appointed Leaders after the last prophet who will help interpret the Quran, Hadith, and establish the virtues of God in society.
The Holy Quran [Shakir 5:55] Only Allah is your Vali and His Messenger and those who believe, those who keep up prayers and pay the poor-rate while they bow.
Shias believe that Imam Ali gave charity during rukkuh. Which is when Muslims bow in prayer.
The Holy Quran [Shakir 4:59] O you who believe! Obey Allah and obey the Messenger and those in authority from among you; then if you quarrel about anything, refer it to Allah and the Messenger, if you believe in Allah and the last day; this is better and very good in the end.
This verse also cites three separate types of obedience. Obedience to Allah (swt), Obedience to the Holy Prophet (s) and to Imam Ali (as), who was the righteous Imam after the Holy Prophet (s) and falls within the meaning of "Ulil Amr". In this connection, Imam Raazi has said that the "Ulil Amr" needs to be infallible just like the Holy Prophet (s).
By saying "la Ilaha Illallah" we declare the unity of Allah, by saying "Mohammad ar-Rasul Allah" we declare the Prophethood of Prophet Muhammad (s) and by saying "Ali un-Wali Allah" Shias declare the guardianship (Wilayat) of Imam Ali (as) and this is necessary because had it not been done, then one-third of verse 4:59 would have been left unpracticed.
All Muslims believe Prophet Muhammad said:
"Of whomsoever I am Maula, 'Ali is his Maula".
Our differences are rooted in our different interpretations of Islam. Only Allah knows who is correct with complete certainty.