What is Monolatry?
The word monolatry is based upon the Greek roots monos, which means one and latreia, which means service or religious worship. Julius Wellhausen seems to have been the first to use the term monolatry, applying it to a type of polytheism in which only one god is worshipped even though the existence of other gods is accepted.Some have also applied it to monotheistic systems in order to underscore the difference between mere belief and religious worship — believing in the existence of one god (monotheism) and actually worshipping that god (monolatry). Thus it's possible to have monolatrous polytheism (believing that multiple gods exist but having exclusive worship and loyalty to one) and monolatrous monotheism (believing that just one god exists and actually worshipping that god).
Monolatry vs. Polytheism
If monolatry involves believing in the existence of multiple gods, why differentiate it from polytheism? What's the difference? The key difference between monolatry and polytheism as it is usually conceived of is the premise that only one of the many gods actually deserves to be worshipped. The reason for this his may be due to a special relationship the god has with the people in question. In the usual forms of polytheism, several or all of the gods deserve worship.
In the ancient Greek religious system, for example, exclusive worship of just one god was forbidden. Some have likened the ancient Greek pantheon to a language, where the meaning of one god can only be understood in the context of all the other gods. Thus ignoring all but one god would, in practice, entail ignoring what made that one god important. This is why the difference between polytheism and monolatry is significant rather than just a minor shift in emphasis.
Monolatry vs. Henothiesm
Closely related to monolatry is the concept of henotheism. Like monolatry, henotheism involves believing that multiple gods exist and focuses on worship of just one of those gods. Unlike monolatry, however, henotheism allows that more than one god may be worthy of worship.
Because of the close similarity, monolatry and henotheism haven't always been used consistently. For example, there are cases where both are applied to the same religious system, with henotheism describing the form of belief while monolatry is used to describe the form of worship and religious activity. While religions in the West typically combine belief in a god and worship of a god, the two are nevertheless logically distinct. Not every person who believes in the existence of a god necessarily worships that god.
Monolatry vs. Monotheism
In practice, monolatry can look a lot like monotheism. While the existence of other gods might be acknowledged, the exclusive focus on worshipping just one god means, on a practical level, that those other gods are ignored. Since no temples are built to honor them, no sacrifices are offered to them, and no prayers are recited to them, they might as well not exist. This is one reason why some scholars are treated monolatry as an intermediate stage between polytheism and monotheism.
Especially in ancient religions, differentiating between monolatrous and monotheistic systems requires careful study of the language used. Monotheistic systems will explicitly deny the existence of other gods and reject alternative religions as false. At most, the gods in other religious systems may be treated as demons or other dangerous spirits. Monolatrous systems, in contrast, will accept that the gods of other systems are indeed gods but advise against worshipping them.
Monolatry in Ancient Judaism
Monolatry may seem strange and foreign to many, but people have probably encountered it without realize it: in the religious texts of ancient Judaism. Today Judaism is recognized as a strict monotheistic faith which helped spawn two other strict monotheistic faiths, Christianity and Islam. The link between Judaism and monotheism is so strong that it's difficult to think of one without the other.
But that's part of the problem: people are so accustomed to seeing Judaism as monotheistic that they miss all the monolatrous references in the oldest texts. The fact that monolatry can look remarkably like monotheism doesn't help matters. Scholars, though, have helped make it clear that there is a mix of monolatry and monotheism in the ancient Jewish texts, with monotheism gradually taking over and replacing monolatry.
Once you know what to look for, the evidence of monolatry is fairly clear and unmistakable. All around are exhortations to worship Yahweh and not other gods, that Yahweh is stronger than other gods, and so forth. This makes far more sense in a monolatrous religious context than in a monotheistic religious context. No one in modern Judaism, Christianity, or Islam argues that people shouldn't worship "other gods" because they deny that any other gods exist in the first place.
The earliest Jews allowed for the existence of other gods, just as other ancient tribes and civilizations did, but the Jews took the unusual step of insisting that foreign gods shouldn't be worshipped at all and, moreover, that they weren't really just manifestations of the same god that they were worshipping. They drew a sharp dividing line between their god and foreign god, just as they drew a sharp dividing line between themselves and foreign peoples.
This clearly wasn't a popular step because the Israelites continually strayed from the true path of worshipping Yahweh alone. The ancient texts are replete with complaints about the people offering up sacrifices to foreign gods and fashioning idols to foreign gods. Those complaints wouldn’t exist if there weren't widespread apostasy that bothered the religious elites and that much apostasy wouldn't exist if the temptations offered by foreign gods and foreign priests weren't strong.
