A Bedouin approached Imam Ali (as) in the Battle of Camel and asked if he asserted that God was one. In answer, Imam Ali (as) said:
To say that God is one has four meanings; Two of these meanings are false and two correct. As for the two incorrect meanings, one is that one should say 'God is one' and be thinking of number and counting. This meaning is false because that which has no second cannot enter into the category of number. Do you not see that those who said that God is the third of trinity [i.e, the Christians] fell into infidelity? Another meaning is to say that so and so is one of this people, namely as a species of this genus or a member of this species. This meaning is also not correct when applied to God, for it implies likening something to God and God is above all likeness.Also Imam Ali (as) said:As for the two meanings that are correct when applied to God, one is that it should be said that God is one in the sense that there is no likeness unto Him among things. God possesses such uniqueness. And one is to say that God is one on the sense that there is no multiplicity or division conceiveable in Him, neither outwardly nor in the mind nor in the imagination. God possesses such unity.
To know God is to know His Oneness.This means that to prove that the Being of God is unlimited and infinite suffices to prove His Oneness, for to conceive a second for the Infinite is impossible.
Ref:
Shi'ite Islam - Page 127-28
Allamah Sayyid Muhammad Husayn Tabatabai
Translated by Seyyed Hossein Nasr
Contributed by Br. Ali Abbas, abbas@seas.gwu.edu