Why didn't early modern Christians object to pagan symbolism in art?

Well, there was actually a movement in the early history of the Church called, "Iconoclasm"-(as noted in one of the previous postings). For approximately 100 years-(I believe either during the 700's or 800's), Churches, namely Eastern Churches-(who were known and are still known for their impressive looking iconography), had actually banned the iconic displays of Jesus, Mary, the Gospel Writers, the Apostles, as well as the various Saints. For a short while, the attitude towards a visible expression of the divine or even the semi-divine was viewed as a distraction from the mystical relationship with the Divine.

The Iconoclasts may have been influenced by the earliest Founders of Christianity, such as Saint Paul, who claimed to have heard the voice of Jesus while on the "Road to Damascus", though never claimed to have seen Jesus. The famous blinding light that Paul claimed to have experienced, never included an image of Jesus. If you are familiar with the Acts and Epistles in The New Testament, Saint Paul constantly preached anti-idolatrous diatribes in public squares, such as Theaters, Marketplaces and even at the Areopagus in Athens.

Saint John-(The 4th New Testament Gospel Writer), also claimed to have heard (a seemingly melodic) voice of Jesus in a cave on Patmos Island in the Aegean sea region. Although Saint John authored the famed "Revelation", it is also the same Saint John who opens his Gospel regarding "the word of God", but not the image of God

It is possible that the Iconoclastic century-(i.e. 700's or 800's) in the Greek Christian East and perhaps to some extent in the Roman Catholic West, were attempting to truly distance themselves from their earlier pagan origins and may have drawn inspiration from the anti-imagery of Saints John (and especially), Paul.

Not a specialist, but my understanding is there were multiple competing trends at work since Christianity has been a thing.

As an answer, I'd stress that the introduction of christianity itself was not a done-deal by any stretch. It's not like entire towns or villages converted overnight - unless they were coerced in doing so. Rather, it was a slow-motion process that went on for centuries. During this process, the Christians basically hopped onto whatever pagan bandwagons they encountered, and actively re-interpreted the local pagan customs whenever they could. Most if not all Christian holy days have pagan roots, for instance. This mixing and re-appropriation continued as far as into the modern age - you can find all sorts of flavorful Christian customs in Africa.

But that's only half the story in my opinion. So I'd like to raise some of the ahistorical points in the question itself at some length. TL;DR version might be: the notion of Christian zealotism is recent and in no way represents, I think, how Christians behaved a few centuries or a millennia ago.

First off, be wary that a devout Church wasn't always a thing - particularly early on. The 10th century, for instance, offers us John XII, who reputably raped nuns and pilgrims, and essentially turned St. Peters into what can arguably be described as a brothel. Hardly a saint.

What more, in the middle ages and in the Renaissance, the idea of a disgruntled King conquering Rome to place someone more accommodating in the Pope's seat was neither weird nor unheard of. There was even a time when there were two popes.

It's also worth noting in passing that, in the early days of the Church, the Pope in Rome was a Bishop like any other for all practical purposes. There were power struggles, and the Pope consolidated its leadership only in the 11th century after the East-West Schism. Before that there were intense disputes, in particular around iconoclasm - which indeed involves one of the movements you might be looking for.

If you dig you'll likely find examples of religious orthodoxy and pushback against paganism once the Inquisition was in full motion. Western Europe was largely Christian by then, and there was no shortage of superstition and zealot behavior.

More so even, perhaps, once the Reformation movements excited swaths of Europe. But by then, the Renaissance was also in full motion. The religious trends at the time were competing with another that saw classical symbolism and iconography make a very strong come back - even the Vatican collects Greek art.

You may find yet more so during the Industrial Revolution and the modern age with when occultist ideas (not to be confused with the early Christian sect) and symbols went mainstream even as the Church dwindled into irrelevance after loosing its lands to Italy, its audience to Marxism, and its international clout to secularism.

In between, religious zealots had no qualms with borrowing symbols from the past. The Jesuits - of all orders - for instance use initials that purportedly were once used to represent Bacchus, the god of wine, who early Christians identified with Jesus. And evangelists had no qualms claiming that the symbols stood for "Isis, Horus, and Seb," and were related to Egyptian sun worship.

On the parish goers' end, it is certainly also worth highlighting that, after the Jesuits got kicked out of Japan, Dutch merchants didn't have much qualms with refraining from missionary activities and stepping on Christian symbols in order to secure business there. So much for Church respect.

The point of this all, of course, is to answer that it's murky: at all times, the Church embraced rather than fought pagan symbols, with the notable exceptions of iconoclasm-related clashes in the context of a power play, and satanist symbolism in the 19-20th centuries in the US - the only modern/developed democracy that is culturally Christian and where secularism isn't mainstream. Before that and in between the two there may have been zealot groups but I can't think of any off the top of my head that had a material impact on the topic.

(Then again, I'm no specialist by any stretch.)

I had a copy of 'Os Lusiadas' (the epic poem written by the Portuguese Camões), with a preface written by a contemporary (XVI century) bishop.

I do not remember the exact words, by the general idea was: "I must inform the reader that the book deals with the ancient pagan religion of Rome and its deities, and in many instances, it is written as if they were true. But as it is not expected that anybody takes this seriously, nor there are any true followers of this old religion which might tempt a christian, it is not a threat to anybody's faith. Besides that, we may also find similar literary forms related to the roman deities in the old Roman classics which we all enjoy."

see, people were smart back there, too. I also guess people never stopped reading classics such as the Odyssey or the Iliad. If you tell teenagers to read Homer, what would be the reason of objecting to a painting of a deity?