The Catechism in Questions & Answers

282. What is the greatest commandment?

When asked which was the “greatest commandment in the law”, Jesus responded with two quotations from the Mosaic Law: “‘You shall love the Lord your God with all your heart, with all your soul, and with all your mind.’ This is the first and great commandment. And the second is like it: ‘You shall love your neighbour as yourself.’ On these two commandments hang all the Law and the Prophets” (Matthew 22: 36-40). The commandment to love God and one’s neighbour is also known as the “dual commandment of love”.

Love for one’s neighbour: see also Question 155.