Q389 - If I Take the Lord’s Name in Vain and Cannot Go to Confession Before Sunday Mass?

It depends. Mortal sin is when you do something with full conscience and freedom, and when what you do is a grievous offense to the holiness of God. So this is a grave sin, like using curses or similar expressions. In such cases, you cannot participate in the community without repentance.

This is a grievous sin. However, when people use the name of God without much thought, this is not a grave sin. Of course, you must strive to abandon this habit, but you can repent and confess on another occasion.

I do not consider this a grave sin when you do it without awareness.