That is not true.
The matter actually has to be grave or thought to be grave. Both satisfy the first condition for a mortal sin to be committed.
If a person commits a sin which is really venial in matter but they think it is of grave matter and they go ahead anyway and commit the act then that is a mortal sin.
Can you provide a church reference that states that the first condition of mortal sin is that the matter must be either grave
or thought to be grave?
I certainly find no such reference in the catechism, which clearly states:
1854
Sins are rightly evaluated according to their gravity. The distinction between mortal and venial sin, already evident in Scripture,129 became part of the tradition of the Church. It is corroborated by human experience.
Especially when the specific gravity of pocketing a piece of candy is not increased simply by the fact that one thinks that it is mortal (unless, as I said, there is a SEPARATE sin of willful disobedience to God involved). To wit: the impact of stealing a piece of candy is unchanged based on whether or not the person thinks it is grave matter, thus the impact is NOT grave (objectively) and, not being grave, the sin of pocketing the candy will ALWAYS be venial, regardless of whether or not someone believes it to be so.
The Catechism further goes on to state:
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.”
The Catechism makes NO provision that a sin THOUGHT to be grave matter becomes grave simply because a person THOUGHT it was grave matter. The condition is that the object actually IS grave matter.
I think, however, I sense where your confusion is coming from. You might be thinking of where the Catechism says:
1860 Unintentional ignorance can diminish or even remove the imputability of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The promptings of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders.
Sin committed through malice, by deliberate choice of evil, is the gravest.
At first glance, that APPEARS to indicate that willful sin is automatically grave. HOWEVER, what this paragraph addresses is how ignorance LESSENS the gravity of a particular sin… not increases it. In other words, when we rank a mortal sin such as missing mass, the person who was ignorant of the sunday obligation is less culpable than the person who is under social pressure not to go, who is less culpable than the person who feels they don’t have time to go, who is less culpable than the person who simply doesn’t feel like going, who is less culpable than the person who doesn’t go to spite God.
But those are ways of determining who is LESS culpable within actions partaining to a particular grave matter.
No such provision whatsoever is given to determine that venial sin is MORE than venial when someone believes that it might be grave matter. The only provision is that ignorance of the gravity of a matter might LESSEN the culpability for acts involving grave matter.
In other words, the Catechism offers no such teaching that a venial sin becomes mortal simply because someone believes that the matter may have been grave when it was not.
So, back to my original question: if the Catechism offers no such statement, can you produce an official document of the church that DOES support that position?
Hope that helps you.