The 7th mitzvah is that we are commanded to swear in G‑d's name whenever necessary — whether to insure something be done or to prevent ourselves from doing something. [We are required to swear in His name] because it exalts, glorifies, and magnifies G‑d.
The source of this commandment is G‑d's statement,1 "And swear in His name."
Our Sages said explicitly,2 "The Torah tells us 'swear in His name' and 'do not swear.' " This means that just as one may not make an unnecessary oath, which is a prohibition, one is commanded to make a necessary oath, which is a positive commandment.
Therefore, one may not swear in the name of any other creation, such as angels or stars. An exception is where the subject [i.e., G‑d] is obviously omitted, such as one who swears in "the truth of the sun," but means "the true G‑d of [i.e., Who created] the sun." It is in this manner that our nation swears in the name of Moshe — in order to gain honor through [mentioning] his name. It is as if the person uttered the oath, "in the G‑d of Moshe," or "in the One Who sent Moshe." However, when the person uttering the oath does not have this in mind, and swears in the name of a created being having in mind that this object is so true that he can swear on it, he has transgressed, and has "placed an object on the same level as G‑d." The Oral Tradition3 explains that "one who 'places an object on the same level as G‑d' is uprooted from the world."
This is the intention of the verse, "swear in His name," i.e., keep in mind that He alone is the True Existence that it is proper to swear by. In the first chapter of T'murah4 our Sages say, "what is the source that one may take an oath to fulfill a mitzvah? The verse, 'And swear in His name.' "
