The Catechism in Questions & Answers

298. What does it mean to violate the commandments of God?

Every violation of God’s commandments is a sin. Sin causes human beings to incur guilt before God. The measure of the guilt arising from this sin can vary. God alone determines the magnitude of guilt. In individual cases it might even be that hardly any guilt is incurred before God as a result of a particular sin.

Relationship between sin and guilt: see Questions 230. and explanation of Question 230.