Charles Ellicott Commentary Matthew 5:33

"Again, ye have heard that it was said to them of old time, Thou shalt not forswear thyself, but shalt perform unto the Lord thine oaths:" — Matthew 5:33 (ASV)

By them of old time — This should be read as to them of old time, as before. Here again, the reference is to the letter of the Law as taught by the Rabbis, who did not go beyond it to its wider spirit. To them, the Third Commandment was simply a prohibition of perjury, just as the Sixth was of murder or the Seventh of adultery. They did not see that the holy name (Leviticus 19:12) could be profaned in other ways, even when it was not spoken.

Furthermore, they expressly or tacitly allowed many forms of oaths in which God's name was not used, with the goal of guarding it from desecration (see Philo, De Special. Legg.). Lastly, out of the many forms sanctioned in this way (as seen here and in Matthew 23:16-22), they selected some as binding and others as not. Through a casuistry that was at once subtle, irrational, and dishonest, they tampered with people’s sense of truthfulness.