This age is the “Antichrist” generation where the enemy is on his last rampage.  We must protect our mind just as devotedly as we would refuse to eat something poisoned.  A poisoned mind is the gateway to destruction.  By the power of Calvary, the Devil has already lost, but we must claim that victory every day.
A great saint of God known as Abbott Serenus, fifth century, stated very clearly that unclean spirits cannot make their way into those whose bodies they are going to sieze upon, in any other way than by first taking possession of their minds and thoughts.1
1Cassian, John, First Conference of Abbot Serenus, 24.