"The mind that is occupied with the thought of God has little room for other thoughts, especially evil ones." St. Ignatius Brianchaninot

Where there is a vacuum, there is an entrance. The more we can keep our minds focused on God and his goodness, the less other thoughts will be able to find a place to root. That is one of the reasons the Desert Fathers would recite a mantra throughout the day like: Lord Jesus Christ, son of the living God, have mercy on me. The Evil One looks for ways to distract us away from God with all kinds of thoughts. It is our choice to decide what our focus will be.  It takes discipline to keep our heart and mind on God. If we close the door, the Lord remains.