1. Solution

12. Hence follows the proposed conclusion about perjury, because to bring in God as witness of what is false is to do irreverence to God: - either by bringing him in as a witness ignorant of the truth and so not omniscient, or bringing him in as willing to testify to what is false and so not completely truthful. In either way irreverence is done immediately to God, against the commandment of the first table, “Thou shalt not take the name of the Lord thy God in vain” [Exodus 20.7]. And so, in either way, if done deliberately, it is a mortal sin.