Iesous ho Chrestos ['Iesous the Good'; Christos in the first century was mistaken by many for the commoner word Chrestos, because by that point the pronunciation of each word was the same], as we know with the hindsight of History, came out on top of the myriad of other deities (Serapis, Osiris, Sabazios, Mithras ...) which made their fashionable journey from the mysterious Orient and secured enthusiastic followings in Greek Rome (never forget that, just as there are more Jews in New York than in any other city in the world, so Rome was the largest Greek-speaking city in the world).
We know this with hindsight; in the first century it far from obvious that this latecomer would displace those who were already firmly established. And foremost among those was Isis.
Isis was an originally Egyptian fertility goddess whose cult was fashioned into a world-beater when she was adopted and hellenised by the Greek monarchs of Egypt (the Ptolemies) who had governed there since the time of Alexander the Great. The last of these Greek dynasts was, indeed, herself an incarnation of Isis (Kleopatra nea Isis philopateira thea). If you want to get a seductive taste of the entrancing power which her cult exercised, read Book XI of the Metamorphoses of Apuleius.
Here is how Isis describes herself:
"Rerum Naturae parens, elementorum omnium domina, saeculorum progenies initialis, summa numinum, regina manium, prima caelitum, deorum dearumque facies uniformis ... cuius numen unicum multiformi specie, ritu vario, nomine multiiugo totus veneratur orbis ..." [Mother of the Nature of Things, Mistress of all the elements, initial Progeny of the centuries, Summary of divinities, Queen of souls, First of the skydwellers, uniform Appearance of the gods and goddesses, ... the whole world adores my deity alone, under a different appearance, a different rite, a different name ...]. Because Isis, as she goes on to explain, does not exclude the other gods and goddesses worshipped in every place upon earth; she is those other divinities. She is simply an expression of all the deities anybody worships anywhere ... or rather, in her age-old Egyptian manifestation, she is the truest such expression and Isis is her truest name. Very modern: inclusive rather than exclusive! [Incidentally, I believe this attitude is very close to Hinduism, where the vast numbers of different divine manifestations are really one single deity. But I'm not an expert ...]
So the Cult of Isis is really a form of monotheism, and does not require anybody else to give up any other deity, for all are one and one is in all. Indeed, after being initiated into her mysteries, the hero of Apuleius' book goes on to become something of a collector of deities and their mysterious cults!
There is a technical name for this approach to the divine: it is called Syncretism.
To be continued.