The first time that the word
atonement is used in scripture, is in Genesis 6, where it's introduced as a concept in the context of the great flood, where God instructs Noah to make that box (ark) and to protect all within with a
covering of pitch. In it's simplest form, atonement is a coating, or a membrane that forms a barrier between two things that are not compatible. It's the separation between the inner and the outer.
An excellent example of this can be seen here>>>
Galatians 4:19 KJV
My little children, of whom I travail in birth again until Christ be formed in you,
When a baby is formed in the womb of its mother a temporary, but physical barrier, is formed between mother and child. The amniotic sac envelops the growing babe as a protective covering and in that sac with the baby is the placenta that allows the bloodstream of the child inside to be nourished and supplied with oxygen from the mother's bloodstream, without ever mixing them. The inside person is within (obviously) dependent upon, and yet still
independent of that outer person. It's miraculous.
While Genesis 6 is the first time we see this "kaphar/kopher" used-- translated later in scripture as atonement covering, it isn't where the idea is first introduced. To see the first form of the concept we have to revisit the first time an atonement was required and to see that we have to look at what Christians commonly refer to as the original sin. That first transgression that caused the separation between God and man.
The tale is told in Genesis 3 and though it's one of the most familiar in all of scripture, many miss the required atonement and subsequent separation that was formed between the inner (spirit) and the outer (host) of man.
Adam and Eve sinned. A transgression occurred and then--
The LORD God made garments from skin for Adam and his wife, and clothed them.
And then they were sent away-- separated from His presence.