St. John gives us three great enemies of the soul: the world, the flesh, and the devil. It is, therefore, always a good idea to know how to challenge these great adversaries. Here's a tip:
The Devil: the Cunning Deceiver
He is the father of lies. He is to be always mistrusted, disdained, and denied. St. Peter describes him as a lion always prowling about looking for someone to devour, and we are exhorted to resist him, solid in our faith. He will certainly tempt and entice with words of honey and half-truths of spice.
How to Combat
A best offence is a great defense, and we have to first know what we are to defend against. The devil's plan is not, as is commonly thought, that we should simply do great evil. He is far more crafty than that. He wants only that we fail to do God's will. So when God says “do good,” the devil either says “do great evil” or “do a little evil.” But when God says “do better,” the devil says “just do good.” And when God says “be perfect,” the devil says “just be a little better.”
Your great weapon here is the sacrament of confession. This followed by the sacrament of the Eucharist is a powerful combo against his lies, because this keeps us in a state of grace, a state of active friendship with God. And the best part about being close friends with God is that he shares with us his Divine Nature (or grace). Sharing in his divine nature means that we walk in his light, which enables us to see more clearly the advances of the enemy and therefore resist them more thoroughly.
The Moral
You are master over yourself. God's grace, his divine nature elevates our human nature to a divinized state that empowers us to see more clearly the lies of the devil. If we should reject this state of life by a deadly sin, we may regain it by humble repentance and the sacrament of confession. But the more we can maintain divine life in our soul the greater will our resources against evil be.