I first encountered this notion on p. 30 of R.S. Sugirtharajah's book The Bible and the Third World: Precolonial, Colonial and Postcolonial Encounters (2001, Cambridge University Press). In reference to the ancient Roman territories of the African continent, he says (with my own emphasis added below) that:
The Christianity which came to Africa was initially hellenized and eventually adapted itself to Egyptian language and culture. This adaptation was facilitated by the infusion of native religion and rituals. It contained elements from non-Christian religions. Augustine, in one of his tracts, mentions that in the fifth century Christians believed that God was Saturn re-christened. The Christianity practised both by sophisticated thinkers like Tertullian and ordinary people had as much in common with the practices of their indigenous religion as with the new faith.
This paragraph is footnoted with: "See Susan Raven, Rome in Africa (London, Routledge, 1993), p. 168."
A look into the cited page of Susan Raven's book Rome in Africa supplies the following (again with my own emphasis):
But the Christianity of the African poor—and even of intellectuals like Tertullian—had much in common with the religion it displaced. The new Semitic God was, for the superstitious Africans, the old Semitic Saturn-Baal writ large: a God of Vengeance to be feared and propitiated, rather than a loving Father. It was 'a transformed popular religion, rather than ...conversion to a new religion'. Indeed, one of St Augustine's tracts suggests that many fifth-century Christians believed that God was Saturn re-christened; and the old god's nickname, senex, the Old Man, may have been applied by extension to Christian bishops, even though the name of Saturn seems to have vanished more than a century earlier.
In the context of the region's religious history, Raven (and Sugirtharajah by extension) seems to be here implying that the Christianisation of the area was, especially at the popular level, an additional veneer of syncretisation which had been occurring since the Phoenicians landed in Africa centuries previously, bringing with them the worship of Ba'al-Hammon, with whom the Roman Saturn was later identified once the Romans had colonised Phoenician Africa.
Of what I have been able to find, the closest to what these authors are talking about, is an argument made by Augustine in his work De Consensu Evangelistarum, "On the Harmony of the Evangelists" (which I have also seen referred to as De Consensu Evangeliorum, "On the Harmony of the Gospels"), specifically in Book 1 (particularly around Chapters 22-25).
In this text Augustine does indeed address syncretisation, but he nowhere, so far as I can tell, accuses Christians of such a practice. He is arguing against the Roman custom or doctrine of ensuring to worship all the deities that one encounters, and he cites certain Roman poets and other writers as equating the God of the Hebrews with their own Saturn or Jupiter.
For the most part, however, he is engaged in a debate positing the apparent contradictions of identifying two or more deities with each other, such as was done, e.g. with Jupiter and Saturn, of whom there exist stories about the two gods actually being each other's enemies. A big part of his argument follows the complaint that it doesn't make sense to claim that the deity worshipped by Jews and Christians is the same as Saturn (or Jupiter), primarily because the former demands exclusive veneration.
As for the divine nickname of Senex, this also receives a mention (at the end of Book 1, Chapter 23) but it is in regard to Carthaginians apparently shying away from calling their deity specifically Saturn and opting rather to call him the Ancient or Old One.
Is there anything in the corpus of Augustine's literature in which this African writer says that Christians themselves during his time (or, really, at any point in history) equated their God with the Roman Saturn (or some other deity)?
Or is what Sugirtharajah quotes from Raven maybe simply an inaccurate representation of statements made in De Consensu Evangelistarum? (It could be that I'm looking in the wrong place among the prodigious number of works which I've heard that Augustine penned.)