They were probably the purest monotheists that classical antiquity produced.
"Roman Society from Nero to Marcus Aurelius"
Samuel Dill
While Mr. Tylor asserts "that no savage tribe of monotheists has ever been known," but that all ascribe the attributes of deity to other beings than the Almighty Creator, it appears in fact that many of the rudest savages "are as monotheistic as some Christians.
"The Faith of the Millions (2nd series)"
George Tyrrell
Vishnuites and Sivaites however are monotheists in the sense that their minor deities are not essentially different from the saints of Roman and Eastern Christianity but their monotheism has a pantheistic tinge.
"Hinduism and Buddhism, Vol I. (of 3) An Historical Sketch"
Charles Eliot