The ancient Greek religion was never a truly unified whole. Indeed, it was so diverse that many academics in the field of Religious Studies speak of Greek 'religions' .
Every region and era had significant differences in their expression and version of the Greek religion when compared with other regions and eras of Greece. The form of the Greek religion in Sparta was very different from the Athenian one, even during the same time periods [examples needed]. Of course, we wouldn't speak of a Greek religion if we didn't have major and important similarities and common ground. However, in this case, I want to focus on the differences to show and explain why I consider the modern Greek religion (especially Reconstructionist Hellenism) to be syncretic and truly modern.
Modern Hellenism is the product of centuries of historical and archaeological research and discovery as well as decades of theological and comparative religious studies. As a result, we have gained an accumulation of knowledge from nearly all eras and regions where the Greek religion flourished. This very fact is what makes Hellenism today a modern and syncretic product. But why?
The ancient Greek religion was alive and naturally evolving. Modern Hellenism is a rebirth from the sum total of known Hellenic information. Due to that, it unites a previously very diverse and regionally autonomous whole of religious versions and practices into roughly one homogenised, and thus syncretic, religious system. Obviously, variations will certainly exist (and they do) because the interpretations of the archaeological evidence and primary sources and the theories expressed by comparative religious studies are subject to opinion and diverging points of view. Also, the variable of actual religious experience modifies further the map of modern Hellenism, birthing almost as many diverse expressions (in its own unique and new way, through the filter of current times) as its ancient forefather.
This results in a religion with the Gods of the Archaic and Classical ages, the practices of the Hellenistic age and the religious understandings of the Alexandrian and later Roman eras. Also, it has the tendency of mixing regional practices, albeit with a prevalent disposition towards the, better documented, Attic forms (creating a peculiar practice of interpretatio attica).
Another difference is the introduction of philosophy in the religion itself as a kind of theology (possibly a remnant of Christian/dogmatic perception of religion, which dominated and still influences the young, modern Hellenism). This, in my opinion, is a faulty approach, since the various philosophers simply expressed and taught to their students (and not all followers of the Greek religion) their personal interpretations and beliefs regarding deity, the world, spirituality, religious practice etc.
The ancient Greek religion was a non-dogmatic, orthopraxic religion. Not an orthodox one, thus not rooted in belief but, rather, in practice and methodological piety. Indeed, in many works , even of philosophers, it appears that the only 'real' - so to speak - requirement was proper, pious participation in the practical side of the religion. In other words, the public and household latreia. Understandably, this means that any given philosopher's theories cannot, in good conscience and historical accuracy, be used as theological treatises applicable to or religiously normative for all modern Hellenists and Hellenes.
It is obvious, I believe, when looking at these great differences even briefly, that modern Hellenism is indeed a product of this era and a result, a 'child' of the sum total of the ancient Greek religion and modern points of view and understandings, as well as, naturally, the influence of abrahamic monotheism and dogmatic approach in theological matters. It is not, therefore, a continuation or evolution, per se, of the old religion. Rather, it is a new 'beast' altogether: a phoenix born from the ashes of the old, dead creature and fed with new substances, creating a new being, not a true child and a continuation of the 'genes'.
Such a mix of information and methodology is, undeniably in my eyes, syncretism.
: Burkert, Walter. Αρχαία Ελληνική Θρησκεία: Αρχαϊκή και Κλασσική Εποχή (translation by Nic. P. Mpezantakos & Aphrodite Avagianou, Kardamitsa Publications, Athens, 1993, - English title: Greek Religion: Archaic and Classic - Original title: Griechische Religion der archaischen und klassischen Epoche, Vol 15 of the Die Religionen der Menscheit series, Verlag W. Kohlhammer, 1977.) Introduction, Chapter 3, p. 41, in the translated Greek edition