He was not a Jew.  There were no Jews attending eastern Orthodox church.  Eastern Orthodox in the region of your ancestors were the Belarusians.

Semonovich is a slightly misspelled patrynomic name in Latin alphabet. Your ancestor had a Russified version of name which should be transliterated in Latin alphabet as Semyonovich . In Cyrillic – Семёнович.  Semyon (Семён / Сямён) was a common name among eastern Slavs.

By the way patrynomic names are  helpful in genealogy research. Imagine you are searching for a father of your ancestor. There’re 5 brothers with the similar surnames who could be  good father candidates for your ancestor. Knowing the name of father through patrynomic names is easy to find paternal ancestry.