39 But, some man will say, “Strong meat is for them that are perfect.” For in many things a relaxation by way of indulgence is allowed to infirmity, although in her utmost sincerity the things be nowise pleasing to truth. Let him say this, whoever dreads not the consequences which are to be dreaded, if once there shall be in any way any lies permitted. In nowise, however, must they be permitted to climb up to such a height as to reach to perjuries and blasphemies: nor must any plea whatever be held out, for which it should be right that perjury should be committed, or, what is more execrable, that God should be blasphemed.
For it does not follow that because the blaspheming is only in pretence and a lie, therefore He is not blasphemed. For at this rate it might be said that perjury is not committed, because it is by a lie that it is committed: for who can be by truth a perjurer? So also by truth can no man be a blasphemer. Doubtless it is a milder kind of false swearing, when a person does not know that thing to be false and believes it to be true, which he swears: like as also Saul blasphemed more excusably, because he did it ignorantly. But the reason why it is worse to blaspheme than to perjure one's self, is, that in false swearing God is taken to witness a false thing, but in blaspheming false things are spoken of God Himself.
Now by so much is a man more inexcusable, whether perjurer or blasphemer, by how much the more, while asserting the things wherein they perjure or blaspheme, they know or believe them to be false. Whoever therefore says that for an imperilled man's temporal safety or life a lie may be told, does too much himself swerve from the path of eternal safety and life, if he says that on that behalf one may even swear by God, or even blaspheme God.
Source: To Consentius: Against Lying (New Advent)