14 All this let us consider, and let us not put our brethren on oath. What do you, O man? At the sacred table you exact an oath, and where Christ lies slain, there you slay your own brother. Robbers, indeed, murder on the highways; but you slay the son in the presence of the mother: committing a murder more accursed than Cain himself; for he slew his brother in solitude and only with present death; but you slay your brother in the midst of the church, and that with the deathless death that is to come!
For think you that the church was made for this purpose, that we might swear? Yea, for this it was made, that we might pray! Is the Table placed there, that we may make adjurations? It is placed there to this end, that we may loose sins, not that we may bind them. But you, if you heed nothing else, reverence at least that book, which you reach forth in putting the oath; and open the Gospel, which you take in hand when you bid swear; and when thou hear what Christ there declares concerning oaths, shudder and desist!
What then does He there say concerning oaths? “But I say unto you, Swear not at all.” And do you convert the Law which forbids swearing into an oath. Oh, what contempt! Oh, what outrage! For you do just the same thing as if any one should bid the lawgiver, who prohibits murder, become himself a party to the murder. Not so much do I lament and weep, when I hear that some persons are slain upon the highway, as I groan, and shed tears, and am horrified, when I see any one coming near this Table, placing his hands upon it, and touching the Gospels, and swearing!
Are you in doubt, I ask, concerning money, and would you slay a soul? What do you gain to match the injury you do to your own soul, and to your neighbour? If you believe that the man is true, do not impose the obligation of the oath; but if you know him to be a liar, do not force him to commit perjury. “But that I may have a full assurance:” says one. Verily, when you have not sworn him, then you will receive a good and full assurance.
Source: Homilies on the Statues (New Advent)