If it doesn't violate a commandment how can it be a mortal sin?

If an action, such as fornication, does not violate a commandment, such as the commandment against adultery, how can it be a mortal sin?
If God intended actions such as fornication to be forbidden in the same way as He forbids adultery and murder, then why was there no commandemnt against it? Why would God use the somewhat specific term “adultery” if He intended it to encompass fornication and any other actions?

Dear D,

The Ten Commandments do not cover every kind of sin; just most of them. We can’t, or at least shouldn’t, second guess God. How He designed His commandments is His business. While He doesn’t mention fornication by name in the Commandments, He does cover it in other places in Scripture.

In his letter to the Galatians St. Paul writes: “Now the works of the flesh are plain: fornication, impurity, licentiousness, idolatry, sorcery, enmity, strife, jealously, anger, selfishness, dissension, factions, envy, drunkenness, carousing, and the like. I warn you, as I warned you before, that those who do such things shall not enter the Kingdom of God.” (Gal 5:19-21)

For a good treatment (too long to include here) of why fornication is sinful, read the answer to the second question on page 66 of “Good News About Sex and Marriage” by Christopher West. It is available through catholic.com or by phone: 888 291 8000.

Fr. Vincent Serpa, O.P.

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