The Athenians were notorious for venerating a great number of gods and goddesses. What may have been religious to the Greeks was "superstitious" to Paul, since they attributed powers to these personified forces of nature which were possessed only by God.

Source:

KJV Defenders Study Bible, by Dr. Henry Morris, Ph.D.
Publisher: Thomas Nelson
Language: English
DEF 10 ISBN 0-529-10444-x
DEF 10-1 ISBN 0-529-10445-8