Jeremias lays down the conditions which justify an oath, and many instances of oaths are found in the epistles of the Apostle of the Gentiles. The words of our Lord [1] do not prohibit the taking of oaths if the requisite conditions be present. They give expression to his desire that all Christians should be so truthful and sincere that it will not be necessary for them to use oaths to confirm the truth of what they assert. The conditions which make an oath lawful are given in the words of Jeremias: [2] " And thou shalt swear, As the Lord liveth, in truth, and in judgement, and in justice."
We swear in truth when we are morally certain that what we assert under oath is according to fact. We are not justified in asserting that to be true which we do not know to be true, and we commit the grave sin of perjury if we swear to what we know to be false.
We swear with judgement when there is a just cause for invoking the testimony of God and it is done with proper consideration and reverence. A just cause will be any matter of some importance for the welfare of either soul or body, whether it be public or private. We are not, therefore, justified in swearing to every assertion which we believe to be true; there must be some special reason for employing the name and authority of God to confirm what we say. However, provided that the other conditions are not wanting, the defect of judgement in swearing will not be more than a venial sin, for it is no more than the idle use of the name of God.
It is an insult to God to invoke his testimony to a sinful act or in furtherance of what is sinful. If this is done, the oath is unlawful on account of the want of justice. Justice, then, in this connection requires that the assertion in an assertory oath should not be sinful, such as a sin of detraction or boasting about past sins. In a promissory oath, that which is promised must be honest and lawful.
There is some difficulty and dispute among theologians about the gravity of the sin which is committed when an oath wants justice. If in an assertory oath the testimony of God is unjustifiably invoked to promote a seriously sinful object, the want of justice in the oath will be gravely sinful. If, on the contrary, the testimony of God does not further the end in view, the want of justice will probably be only venially sinful, because the assertion is true, as is here presumed, and the irreverence committed against God by invoking his testimony even to a gravely sinful act does not seem to many to be
