Sun worship was a type of religion, a usually primitive superstition religion centering on the sun.
The Roman Empire on Earth had no sun worshipers. Upon finding the seemingly sun worshipping Children of the Son on 892-IV, a planet with a civilization closely paralleling Rome, Leonard McCoy showed surprise at this striking difference. Spock furthermore thought it illogical for such a religion to develop a philosophy of total brotherhood, as appeared to have happened. However, the inhabitants of 892-IV did not turn out to worship the sun after all. (TOS: "Bread and Circuses")