You must remember that the modern notion of History is not the ancient one. Ancient historians were expected to tell the tale of their ancestors, not be completely bound by verisimilitude.
A good example is Tacitus, who wrote on the Germanic tribes to juxtapose their liberty to Roman torpor, or Polybius who wrote on Rome as a way to encourage the Greeks to emulate their practices, which he considered to be based on virtue.
It was standard practice to invent speeches, make references to events and basically write around a theme or a purpose. Livy wrote to support the return to earlier Romanitas, not to give a factual account. Tacitus took the dry factual accounts, the Annals of Rome, and rewrote it into a History, the Annals as we know them today, a literary work in fact. We see this same Hellenistic structure clearly mirrored in both Luke and Acts.
The idea represented, the point of the work, is more important than whether it represents what factually occurred. This is the misunderstanding of the modern world. Owen Barfield wrote on this, the Ancient Unities of concepts. To the ancients, whether something is just an account of something, does not make it more 'real' or correct. The gist or spirit should be the same. So when Calgacus makes a speech to his troops, it doesn't matter if he did not say the exact words, but the spirit thereof should be similar, should sketch or help the reader experience the event. After all, if we look at the intellectual traditions of Hellenistic times, they were very much interested in Forms, Ideas, abstract expression and sceptical of our abilities to represent reality otherwise. This is clearly mirrored in their historical works. The early Church works from this tradition, as Stephen's speech in Acts makes clear. This is not 'wrong' per se, but can perhaps be thought of how we would make a historical fiction today, a retelling of a true story. The exact details are hardly the point here, in fact they are beside the point.
So to Christianising previous ideas, these were mostly via symbol. To take an example, think of the Statue of Liberty. This is the Roman goddess Liberty standing on chains, but to the French who built her, she did not represent the goddess, but an abstract conception of Liberty and abolition of slavery, expressed via the culturally appropriate symbolism of a goddess. In the US, this came to be reinterpreted first into the 'melting pot' idea of immigrants and then into a general symbol of Americaness. Nowhere did the Roman goddess come into play here at all, it was entirely a secular symbol for secular ideas, expressed thereby.
Likewise, Christians expressed Christian ideas via Pagan symbolism, as this was the culturally appropriate symbols of their time. So we see that the idea of Jesus as the Ultimate God, is expressed by the solar quadriga, but this does not show Jesus as a sun god, but borrows from pagan conceptions to express it. Or we see Saints with accoutrements of pagan gods or the Virgin Mary represented as an ideal of motherhood by the semiotics used to represent it in that time. Why is this strange? If a cartoon in the newspaper presents a political figure as Jesus, we understand what is meant, not that that figure is suddenly declared to be a god, nor that Christianity somehow became absorbed into his representation by it.
How do you convert people to your ideals? You make them understand it, you present it in a way that they understand, you use their known symbols and language forms. To expect Christianity to completely ignore the prevailing culture of the period is frankly ridiculous. It is for this same reason why Robespierre created a 'Goddess of Reason' cult, or Marxism is expressed in terms of dialectic: You first use what people know to express the new, to make them understand the basics, and then build from there.
