All the Manifestations of God came with the same purpose,
and they have all sought to lead men into the paths of virtue. Yet we, their
servants, still dispute among ourselves! Why is it thus? Why do we not love one
another and live in unity?
It is because we have shut our eyes to the underlying principle
of all religions, that God is one, that He is the Father of us all, that we are
all immersed in the ocean of His mercy and sheltered and protected by His
loving care. 
- ‘Abdu’l-Baha  (From a talk; ‘Paris Talks: Addresses given by
'Abdu'l-Bahá in Paris in 1911-1912’)