Sermons of the Curé D'Ars: Over and Above These is Evil

"The name of God is so holy, so great, and so adorable that the angels and the Saints, St. John tells us, say unceasingly in Heaven: 'Holy, holy, holy, is the great God of hosts; may his holy name be blessed for ever and ever.' When the Blessed Virgin went to visit her cousin Elizabeth and the saintly woman said to her, 'How happy you are to have been chosen to be the mother of God!' the Blessed Virgin replied to her: 'He that is mighty hath done great things to me, and holy is His name.'
...St. Thomas Aquinas tells us that it is a serious sin to pronounce the Name of God in vain, that it is not a sin like other sins. In other sins the light nature of the matter diminishes the seriousness of and the malice in them, and quite often what could be a mortal sin is only a venial one. For instance, larceny is a mortal sin,  but if it is larceny of something very small, like a couple of pennies, then it will be a venial sin only. Anger and gluttony are mortal sins, but slight anger or a little gluttony are only venial sins.
IN REGARD TO SWEARING, however, it is not the same thing at all; here the lighter the matter, the greater the irreverence, as if a person were to ask the king to serve as a witness to some trifle, which would be to make a fool of him and to belittle him. Almighty God tells us that anyone who swears by His Name will be sternly punished...the Holy Scripture tells us again that whoever is accustomed to swearing, his house will be filled with iniquities and the curse will never leave the house until it has been destroyed...When you want to confirm something say: 'that is' or 'that is not.' 'Yes' or 'No'. 'I did it' or 'I did not do it.'
Everything you say over and above that comes from the devil."