I assume the term “pagan” was there used to mean pre-Abrahamic, meaning before Judaism, Christianty, or Islam. Though that usually implies polytheism, all it etymologically means is “folksy” and “rustic.” Peasant religion, in other words. There was such a thing as monotheism prior to Jews. See Amun-Ra in ancient Egypt.
You are correct to emphasize the similarity between Platonic philosophy and Christian theology. Many have commented on the God-like quality of “the Good.” Some theorize one of the reasons Socrates was killed for impiety and corruption of the youth is because he taught monotheism.
>>I assume
Your assumption is wrong because the direction is wrong. If one reads Plato’s Republic/Symposium/Dialogues etc, he uses inductive thinking to try to identify what God is.
Paganism has pre-defined gods that are worshiped in established ways.
The only similarity that Platonic philosophy has with Christianity is the number 3. Well, Plato happens to love the number 3 - there are three elements of the ideal society in the ‘Republic’, there is the tripartide soul, there are his three levels of reality. He loves the number three. Does it have to do with the Trinity? No.