What inscription did Paul find on an altar in Athens?

Answer: To an unknown god

Paul told the Athenians, "What you worship as something unknown, I now proclaim to you."

“Then Paul stood up in the meeting of the Areopagus and said, “Men of Athens, I see that in every way you are very religious. For as I walked around and examined your objects of worship, I even found an altar with this inscription: TO AN UNKNOWN GOD. Therefore what you worship as something unknown, I now proclaim to you.”— Acts 17:22-23 (BSB)