God is revealed as perfect interior holiness.
Isaiah 6:3
"Holy, holy, holy is the Lord of hosts!" they (the Seraphim) cried one to the other.
The basis for the belief in the Immaculate Conception of Mary
can be found in the Biblical revelation of holiness and the
opposite of that state, sinfulness.
No sin or anything tainted with sin can stand in the face
of the holiness of God. "Enmity" is that mutual hatred between
Mary and sin, between Christ and sin.

Genesis 3:15
      I will put enmity between you (the serpent, Satan) and
           the woman (Mary), and between your offspring (minions
           of Satan) and hers (Jesus); He will strike at your head,
           while you strike at his heel.