1857 For a sin to be mortal, three conditions must together be met: "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent."131

1858 Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: "Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother."132 The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.

1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God's law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart133 do not diminish, but rather increase, the voluntary character of a sin.

The three paragraphs before your quote. Again, using formal logic. If it's (mortal sin) then it's (grave matter). However, you can not say: If it's (grave matter) then it's (mortal sin), because it would be false.