Objection 1. It would seem that justice, judgment and truth are unsuitably assigned as the conditions accompanying an oath. Things should not be enumerated as diverse, if one of them includes the other. Now of these three, one includes another, since truth is a part of justice, according to Tully (De Invent. Rhet. ii, 53): and judgment is an act of justice, as stated above (Question 60, Article 1). Therefore the three accompanying conditions of an oath are unsuitably assigned.
Objection 2. Further, many other things are required for an oath, namely, devotion, and faith whereby we believe that God knows all things and cannot lie. Therefore the accompanying conditions of an oath are insufficiently enumerated.
Objection 3. Further, these three are requisite in man's every deed: since he ought to do nothing contrary to justice and truth, or without judgment, according to 1 Timothy 5:21, "Do nothing without prejudice," i.e. without previous judgment [Vulgate: 'Observe these things without prejudice, doing nothing by declining to either side.']. Therefore these three should not be associated with an oath any more than with other human actions.
On the contrary, It is written (Jeremiah 4:2): "Thou shalt swear: As the Lord liveth, in truth, and in judgment, and in justice": which words Jerome expounds, saying: "Observe that an oath must be accompanied by these conditions, truth, judgment and justice."
I answer that, As stated above (Article 2), an oath is not good except for one who makes good use of it. Now two conditions are required for the good use of an oath. First, that one swear, not for frivolous, but for urgent reasons, and with discretion; and this requires judgment or discretion on the part of the person who swears. Secondly, as regards the point to be confirmed by oath, that it be neither false, nor unlawful, and this requires both truth, so that one employ an oath in order to confirm what is true, and justice, so that one confirm what is lawful. A rash oath lacks judgment, a false oath lacks truth, and a wicked or unlawful oath lacks justice.
Reply to Objection 1. Judgment does not signify here the execution of justice, but the judgment of discretion, as stated above. Nor is truth here to be taken for the part of justice, but for a condition of speech.
Reply to Objection 2. Devotion, faith and like conditions requisite for the right manner of swearing are implied by judgment: for the other two regard the things sworn to as stated above. We might also reply that justice regards the reason for swearing.
Reply to Objection 3. There is great danger in swearing, both on account of the greatness of God Who is called upon to bear witness, and on account of the frailty of the human tongue, the words of which are confirmed by oath. Hence these conditions are more requisite for an oath than for other human actions.
Source: Summa Theologica (New Advent)