Today, Jesus goes on commenting the Commandments. The Israelites had a great respect for the name of God, a fearful veneration, for they knew that names refer to persons, and God deserves all respect, all honor and all glory, by thought, word and deed. This is why —bearing in mind that swearing is to place God as witness to the truth of what we are saying— the Law commanded them: «Do not break your oath; an oath sworn to the Lord must be kept» (Mt 5:33). But Jesus comes to perfect the Law (and, therefore, to perfect us too by following the Law), and goes a step further: «Do not take oaths. Do not swear by the Heavens (...), nor by the earth (...)» (Mt 5:34). We cannot actually say that to swear is bad, per se, but to make an oath legitimate a few conditions are needed first, such as a fair, grave and serious cause (for instance, a lawsuit), and that your oath be true and good.
But the Lord says even more: «Say yes when you mean yes and say no when you mean no» (Mt 5:37). That is, He invites us to live in truth on every instance, to conform our thinking, our words and our deeds to the truth. But, the truth is what? This is the great question, already formulated in the Gospel, during the judgment against Jesus, in Pilate's own words, which so many thinkers, throughout time, have been trying to answer to. The Truth is God. Whoever lives by pleasing God, by abiding by his Commandments, lives in Truth. The Rector of Ars says: «The reason why so few Christians act with the exclusive purpose of pleasing God is because they are immersed in the most terrible ignorance. O God, how many good deeds are lost for Heaven!». It would be good to ponder over it.
We must develop ourselves, to read the Gospel and the Catechism. And afterwards, we must live by what we have learnt.