Because the Almighty has revealed himself to us as our God and Lord, we must not place anything above him or consider anything more important or give any other thing or person priority over him. To know God and to serve and worship him has absolute priority in our life.
God expects us to give him our full faith; we should place all our hope in him and direct all the strength of our love toward him. The commandment to love God is the most important of all commandments and the key to all the others. That is why it stands at the beginning of the Ten Commandments.
(From the Catholic Youth Catechism, questions 351-352)