Again you have heard that it was said to the first ones: ‘Do not swear falsely, but pay your oath to the Lord.’
In this passage Jesus refers to one of the commandments of the Old Testament that concerns the matter of oaths and promises. Making a false oath was forbidden (Lev. 19:12), and people were obligated to fulfill the promises they had made, especially those oaths that were made in the name of the Lord. An oath in God’s name was regarded as extremely serious, and breaking it was considered sin. It was the custom of the people of that time to swear by various things: by God’s name, by heaven, or by earth. Jesus addresses this issue, then adds His command concerning oaths, which requires a person to be so honest that there is no need for an oath.