Truth be Told: Basics in Catholic Apologetics is a good book for learning to defend parts of the Catholic faith to those who challenge it. It will also help you learn more about your faith! The book is aimed at teenagers, but anyone with youth in their lives can benefit from reading it.
God is an exchange of love. Why is it a mystery? Because we can’t know it unless God reveals us. God is one substance and three persons. God is love. All three persons in the Trinity are the same divine substance.