From the Compendium of the CCC vatican.va/archive/compendium_ccc/documents/archive_2005_compendium-ccc_en.html
THE SECOND COMMANDMENT:
YOU SHALL NOT TAKE THE NAME OF THE LORD YOUR GOD IN VAIN
- How does one respect the holiness of the Name of God?
One shows respect for the holy Name of God by blessing it, praising it and glorifying it. It is forbidden, therefore, to call on the Name of God to justify a crime. It is also wrong to use the holy Name of God in any improper way as in blasphemy (which by its nature is a grave sin), curses, and unfaithfulness to promises made in the Name of God.
- Why is a false oath forbidden?
It is forbidden because one calls upon God who is truth itself to be the witness to a lie.
“Do not swear, whether by the Creator or by any creature, except truthfully, of necessity and with reverence.” (Saint Ignatius of Loyola)
- What is perjury?
Perjury is to make a promise under oath with the intention of not keeping it or to violate a promise made under oath. It is a grave sin against God who is always faithful to his promises.
- When does one commit a mortal sin?
One commits a mortal sin when there are simultaneously present: grave matter, full knowledge, and deliberate consent. This sin destroys charity in us, deprives us of sanctifying grace, and, if unrepented, leads us to the eternal death of hell. It can be forgiven in the ordinary way by means of the sacraments of Baptism and of Penance or Reconciliation.
- When does one commit a venial sin?
One commits a venial sin, which is essentially different from a mortal sin, when the matter involved is less serious or, even if it is grave, when full knowledge or complete consent are absent. Venial sin does not break the covenant with God but it weakens charity and manifests a disordered affection for created goods. It impedes the progress of a soul in the exercise of the virtues and in the practice of moral good. It merits temporal punishment which purifies.