The question of whether a Christian is permitted to swear an oath was hotly debated for many centuries.  The Lord's words in Matthew 5, 33-37, seem to preclude this altogether.  The English translation used at Mass is written this way: "I say to you, do not swear at all; not by heaven, for it is God's throne, nor by the earth, for it is his footstool, nor by Jerusalem, for it is the city of the great King.  Do not swear by your head . . . Let your 'Yes' mean 'Yes,' and your 'No' mean 'No.'  Anything more is from the evil one."  However, a better English translation of the text might be: 'Do not swear at all either by heaven . . . or the earth . . . or by Jerusalem.'  Let us take into account what St. Jerome (d. 430) said on this issue in his commentary on the Gospel of Matthew:

"He who swears, worships or loves that by which he swears.  In the law, it is commanded that we swear only by The Lord our God (Deuteronomy 6, 13-14).  But the Jews swore by the angels, the city of Jerusalem, the temple, and the elements, and they venerated physical creatures with the honor and worship due to God.  Consider that the Savior, here, did not forbid swearing itself, but only swearing by heaven, earth, Jerusalem, and one's own head.  [Permitting oaths] was a concession in the law granted [to the Jews] as to little children, as in the case of the concession granted to them to sacrifice victims to God, lest they offer sacrifices to idols.  Thus, they were permitted to swear oaths by God, which they rightly should not do, but it was better for them to do this than to swear by demons."