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)
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Paragraph 2150
2150. The second commandment forbids false oaths. Taking an oath or swearing is to take God as witness to what one affirms. It is to invoke the divine truthfulness as a pledge of one's own truthfulness. An oath engages the Lord's name. "You shall fear the LORD your God; you shall serve him, and swear by his name."81
Paragraph 2151
2151. Rejection of false oaths is a duty toward God. As Creator and Lord, God is the norm of all truth. Human speech is either in accord with or in opposition to God who is Truth itself. When it is truthful and legitimate, an oath highlights the relationship of human speech with God's truth. A false oath calls on God to be witness to a lie.
Paragraph 2163
2163. False oaths call on God to be witness to a lie. Perjury is a grave offence against the Lord who is always faithful to his promises.
Paragraph 2164
2164. "Do not swear whether by the Creator, or any creature, except truthfully, of necessity, and with reverence" (St. Ignatius of Loyola, Spiritual Exercises, 38).
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