Conscience 
The word is derived from the Latin conscientia, which is a compound of the preposition con and scio, meaning "to know together," "joint knowledge with others," "the knowledge we share with another." It stems from the same root as consciousness, which means "awareness of." Conscience is an awareness restricted to the moral sphere. It is a moral awareness. The Greek equivalent in the NT is syneidesis, a compound of syn, "together," and eidenai, "to know," that is, to know together with, to have common knowledge together with someone. The German Gewissen has the same meaning. The prefix ge expresses a collective idea, the "together with," and wissen is "to know."
In
the Bible. The word "conscience" does not appear in the OT. However,
the idea is well known and is expressed by the term "heart." It
appears at the very dawn of human history as a sense of guilt with Adam and Eve
after the fall. We read of David that his heart smote him (II Sam. 24:10). Job
says: "My heart shall not reproach me" (Job 27:6). And Pss. 32:1-5
and 51:1-9 are the cries of anguish of an aroused conscience.
The conscience is that part of the human though
which the holy Spirit can speak and plead with us.
The
Babylonians, like the Hebrews, identified conscience with the heart. The
Egyptians had no specific word for conscience but recognized its authority, as
is evident from the Book of the Dead. The early Greeks and Romans personified
conscience and depicted it as fiendish female demons called Erinyes and Furies
respectively.
The
word syneidesis or "conscience" appears thirty times in the NT,
nineteen times in the writings of Paul, five times in Hebrews, three times in
the letters of Peter, twice in Acts, and once in the Gospel of John.