Easton's Bible Dictionary
On his second missionary journey Paul visited this city (Acts 17:15; Compare 1 Thessalonians 3:1 ), and delivered in the Areopagus his famous speech (17:22-31). The altar of which Paul there speaks as dedicated "to the [properly "an"] unknown God" (23) was probably one of several which bore the same inscription. It is supposed that they originated in the practice of letting loose a flock of sheep and goats in the streets of Athens on the occasion of a plague, and of offering them up in sacrifice, at the spot where they lay down, "to the god concerned."
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain.
Easton, Matthew George. Entry for 'Athens'. Easton's Bible Dictionary. https://www.studylight.org/dictionaries/eng/ebd/a/athens.html. 1897.