St Thomas Aquinas is one of the key saints to become a doctor of the church, and wrote extensively about theology: much of our understanding of who God is and of our place in relation to him comes from St Thomas' work. He is in particular known for his encyclopaedic length 'Summa Theologica'. The Summa is the foundation for much of our doctrinal thought and also his thinking is quite practical:
"Humans do not have a natural tendency to commit evil or sinful acts. Instead, any wrong or sinful acts that may be carried out are due to mistaking a wrong act for a right act. When the wrong act is chosen it is due to a fault in the reasoning of the individual. Just because an action may seem like the right one doesn’t necessarily mean that it is. Any action that jeopardises humans' relationship with God is a wrong action."
He was also a somewhat remarkable person! His family were very distressed when he left their affluence and entered religious life - even to the extent of paying for prostitutes to visit him in his cell to try and entrap him into abandoning his vow of celibacy. He drove them off with a burning stick from his fireplace. He travelled widely, living in many parts of Europe and always teaching and writing - he is the patron saint of all Catholic education.
Towards the end of his life he had a mystical experience in chapel, and immediately said that all he had written was dull compared with the truth he experienced in that vision. He never wrote again, devoting himself to preparing for his death which occurred a year or so later.