Video from The Thomistic Institute
"In this Conversation from Quarantine, Fr. Basil Cole, O.P. explains the fundamental teachings on angels from their creation, nature and function to their appearances in the Old and the New Testaments.
The demons, while metaphysically the same as angels (pure spirits), were cast out of heaven for failing the trial that God had given them. We discover something about their trial by looking at the trial of Adam and Eve; they had to humbly let go and rely on God to bring them into heaven. The demons wanted to do it on their own terms and fell on account of their pride.
God decided to let the demons have some kind of dominion and influence in this world to try us so that we can grow in virtue and also to give us the grace to overcome these trials.
The demons try to attract us to mortal sin (or venial sin which will lead to mortal sin) through our imagination, memory and anything in this world at their disposal. Our guardian angel, however, attempts to influence us by the same means towards a life of virtue and holiness.
Fr. Basil recommends the following means in order to combat the demonic influence that we encounter in our daily lives: prayer (particularly in this context, to our guardian angels), receiving the Sacraments, devotion to the Blessed Virgin Mary, Holy Water, Sacramentals, statues and medals. Furthermore, educating children to avoid all things that have to do with the occult is crucial." from the video introduction.
For further study on this topic, access the following Aquinas 101 materials:
Course 3, Lesson 41: Angels and Demons
Course 3, Lesson 42: Angelic Knowledge and Choice