It is obvious that Paul was an intellectual giant and therefore more than able to debate with other intellectual persons. Yet he was more than a person with a great mind - he also possessed an intense desire for God to be glorified and a longing for people, both Jews and Gentiles, to be converted. And he was prepared to stand for Jesus by himself. As far as we know, Paul was the only Christian in Athens during the days described in verses 16-21.
The Athenians were used to discussing new ideas, but they had never heard a message like the one Paul was advocating. Some thought his ideas were nonsense. Others, despite their intelligence, assumed that he was speaking about two gods, one called Jesus and the other the resurrection. Their curiosity made them want to find out more before they moved on to the next new idea.
Given the opportunity to speak at the gathering of philosophers in Athens, Paul embraced it and proceeded to tell the philosophers and other intellectuals about the true God. The apostle was fully aware that his listeners had little idea about their Creator, and proceeded to inform them of some basic facts. They heard that God was the sovereign creator, the source of life, the controller of history, the Lord who wanted his creatures to know him since each of them are made in his image.
They also heard that all humans were descended from one man and that all of them had turned away from God. Moreover the place where they were, where intellectual pursuits was valued, was actually a place of ignorance because they had devised and crafted gods made in their image and imagined that was what God was like.
They also heard that the true God commanded them to return to him in repentance because the Day of Judgement has been fixed and that Jesus would be the judge, with his resurrection being proof to all of them that their judgement will take place.
The response was predictable and not much different to that given by less intelligent audiences - some laughed, some wanted to hear more, and some believed in Jesus. So while Paul may have been the only Christian in Athens in verse 21, he had caused others to become believers by verse 34. His efforts were honoured by God and a church was founded.
Paul must have told the thrust of his sermon to Luke several years later. Evidently Paul remembered well what he had said. Maybe he sensed the power of God as he spoke about him and saw that power leading some to faith even although they were surrounded by powerless idols. Perhaps this was the type of sermon he preached to Gentile gatherings composed of intellectuals when he had the opportunity.
Paul was about to go to Corinth, and later when writing a letter to the church in Corinth he would say that not many wise were called. No doubt, he was glad to use 'not many' rather than 'not any', and maybe his converts in Athens came to his mind as he wrote those words.