This question of Hinduism and monotheism reflects certain preconceptions that should be carefully examined. First is the idea that monotheism is a higher form, if not the highest form of religion, which is both debatable and controversial.
Second, if Hinduism is not monotheistic, the general implication is that Hinduism must be polytheistic, meaning heathen, pagan, primitive, superstitious and idolatrous. This view also implies the superiority of monotheism over different formulations of the sacred that may have their own value.
Third is the implication – which unfortunately many Hindus take – that if Hinduism is accepted as another monotheistic faith that Hinduism will be afforded more respect in the world in which monotheistic faiths predominate. Yet one could propose to the contrary that turning Hinduism into another monotheistic faith could as likely make Hindus more vulnerable to conversion by faiths claiming to be more purely monotheistic than Hinduism could ever be.
Moreover, monotheism has a considerable baggage and a history that has often been intolerant, oppressive and violent. There are many important thinkers, philosophers, scientists and artists in the West who have long criticized or rejected monotheism. And many great mystics and yogis from throughout the world have also questioned the superiority of monotheism, its practices and institutions, over spiritual approaches to Self-knowledge and Self-realization not tied to one belief system or another.