Final answer:
Paul saw a temple to an unknown god in Athens, which he used as a starting point to introduce the Athenians to the Christian God.
Step-by-step explanation:
The city where Paul saw a temple to an unknown god was Athens. This event is recounted in the Bible's New Testament book of Acts, specifically in Acts 17:23. While Paul was in Athens, he noticed an altar with the inscription 'To an Unknown God.'
Seizing the opportunity, Paul then addressed the Athenians about their religious beliefs and introduced them to the Christian God, thus using their own altar as a starting point for his discourse on the nature of the God of Christianity.