The idea of religious conversion is a trickier one to unpack than it might initially appear. Nowadays, particularly within the framework of a monotheistic society (i.e. a society in which the primary religions are focused on the worship of one supreme deity, such as Islam and Christianity), we generally talk about conversion as the process of abandoning one set of beliefs in favour of another, during which we lose identification with the former religion and identify wholly with another. To put it in blunt terms, we tend to think of conversion as a moment of spiritual enlightenment, separating the before and after of our beliefs. When we think of converting between monotheistic religions, this process is complicated by the notion of the one true deity. Most modern religions (by which I mean religions which developed after the ancient world) specify in their dogma that the believer must abandon all their other religious affiliations and worship only the god of their chosen religion.

For example, from the King James Bible:

Unto thee it was shown, that thou mightest know that the LORD he is God; there is none else beside him. (39) Know therefore this day, and consider it in thine heart, that the LORD he is God in heaven above, and upon the earth beneath: there is none else. (Deuteronomy 4:35,39)

And similarly, one of the core tenets of Islam, the shahadah, means that those who adhere to Islamic belief must swear under oath that there is only one true god, Allah.

So, we can see here that, apart from the necessary exceptions to the rule – people who are torn between two religions, for example, or those raised in multi-faith households – conversion as we tend to think of it in modern terms necessitates at least in part a rejection of one’s former beliefs in order to fully embrace one’s newer religious identity.

Conversion within a polytheistic society, such as those of Ancient Greece and Ancient Egypt, to give the examples you’ve requested, did not function in this way. This is because the fabric of religious life was different. Firstly, religion was not viewed as a separate or discrete element of daily life, but rather a lens through which one viewed the entire world. Religion did not consist of doctrine, but rather ritual, and this formed a hugely important part of how society functioned. Ritual, for example, was tied to just about every event you can think of, from the important life events such as birth, marriage and death to the more mundane or quotidian aspects, such as entering a home, going into town, or cooking a meal. Different gods were associated with almost every action or event, and so religion was not a case of ensuring that you went to temple when you were supposed to, or attended festivals; it was literally the way of life.

Part of the way this functioned was the plethora of gods. Nowadays, when we think about Greek religion, we tend to think about the main gods. We think of Zeus, Hera, Aphrodite, Apollo – the gods whose popularity still remains current. However, there were absolute multitudes of other deities whose worship was more discreet or less widespread. To give a few examples, Hades was not the only ‘god of the Underworld’, but rather one of myriad chthonic (i.e. ‘of the Underworld’) deities, including Melinoe, Nyx, Hecate, Hypnus, Thanatos and many others. Each god governed a specific realm, with many gods’ roles overlapping; Hades, Hypnus and Thanatos were each associated with different elements of death, for example. This meant that, in some respects, it was up to the discretion of each worshipper to dedicate offerings and worship to certain gods, and the gods favoured by one individual might differ from the gods favoured by another, even though they were both following the same religious framework.

To further complicate things, Greek religion was not simply one broad spectrum of gods, but a complex system of cults and worship. We tend to think of the word ‘cult’ as being something negative today, implying brainwashing and rigid adherence to doctrine which severely impacts freedom of choice. This wasn’t what cults were in ancient religion; rather they were a series of rituals linked to particular dieties, through which devotees could access different levels of religious experience. For example, those initiated into the Eleusinian Mysteries performed highly secretive rituals at religious locations in order to gain greater insight into and subsequent advantages when they entered the afterlife. This didn’t mean that those initiated into these cults had given up on the broad spectrum of Greek religion and chosen to focus solely on the cult; it was simply another layer to their lived experience.

Therefore, conversion in ancient religion was not exactly – or at least, not necessarily – a case of turning away from an old system of belief and embracing a new one, but rather incorporating a secondary belief system into the existing one. This was also applicable across pantheons, largely due to the multicultural nature of the ancient world. As civilisations mixed and migration and conquest occurred on a greater scale, religions became ever more syncretic. Syncretism here means the process of several religions becoming merged through cultural contact and forming a new religious framework, such as in the case of Vodou, which is a mixture of Catholic beliefs and indigenous ritual that occurred when West African slaves were forced to convert to Christianity by their oppressors. In an ancient context, it was not always or even often a result of forced conversion, but rather a more organic mingling of pantheons. It could also be used for political effect, such as an attempt to placate conquered peoples; if the conquering force agreed to adopt the local gods into their own pantheon so that the local people could retain their religious beliefs, for example, it went a long way to reducing resistance and increasing the chance of a lasting conquest. It worked the other way, too; often a conquering force would adopt the deities of the lands they conquered as a show of victory, to indicate that they had united their pantheons.

This was often successful because gods were not singular beings, but complex, multifaceted entities with multiple aspects. In other words, one god – taking Athena as a Greek example – performed multiple functions, which could be denoted by the particular use of her name, or epithet. For example, Athena Polias, meaning Athena of the city, was associated with her links to the city of Athens, focusing on her military expertise and keen intelligence. Athena Parthenos, meaning Athena the maiden / virgin, focused on her role as an enforcer of sexual modesty and chastity. These were not different gods, but ways of invoking the specific nuanced functions of a single deity.

When syncretism comes into play with this established practice, the epithet of a god can often occur as a sort of hyphenation of two deities from different pantheons. This is because God A from Religion 1 and God B from Religion 2 were often viewed not as a separate deity, but as different aspects of the same god. Isis is actually one of the best examples of this. When Egyptian religion became more and more syncretic, Isis’ cult in Rome grew in popularity. Her cult forms a key part of one famous Roman novel, Apuleius’ Metamorphoses, known today as The Golden Ass. In this text, the protagonist, Lucius, experiences a spiritual revelation when a goddess reveals herself to him and promises to save him from his predicament if he joins her cult. Rather than telling him ‘I am [Goddess name]’, she says:

In one place the Phrygians […] call me Pessinuntine Mother of the Gods, in another the autochthonous people of Attica call me Cecropian Minerva, in another the sea-washed Cyprians call me Paphian Venus […] (11.5)

By using multiple goddesses from different cultures and portraying them as being valid aspects of herself, Isis here is not elevated in status above other religious frameworks; rather, she is included in them. When Lucius then converts and agrees to initiate himself in the rites of her cult, he does so with the belief that he is not abandoning the Roman gods, but choosing a particular form of worship of one aspect of one deity. Here, we can see that the notion of multiple deities across pantheons having the same function or being linked to the same sphere of daily life was not a cause for conflict, and it did not necessitate people to choose between one deity or the other; rather they could choose to worship both deities by viewing them as aspects of the same being, or they could choose to worship one aspect over another.

For this reason, it is much more difficult to pinpoint the notion of conversion as a discrete process, i.e. a process of abandoning one religion and embracing another, in the ancient world. Conversion in their religious framework was far more fluid and one could initiate oneself into particular cults and still adhere to the same religious beliefs as before, simply with new layers of meaning.