I got the definition of "promise" from wikipedia:
"A promise (also called troth) is a psychological contract indicating a transaction between two persons whereby the first person undertakes in the future to render some service or gift to the second person or devotes something valuable now and here to his or her use. A promise may also refer to any kind of vow or guarantee.
Promises and religions
Religions have differing attitudes towards promises.
Main article: Oath
In Christianity, a distinction is made between simple promises and oaths/vows, with only the latter being seen as involving God, either as witness to the promise or recipient of it, although God sees the simple promises too.
The act of making a solemn oath may be done on one's own, but certain oaths or vows, especially when it affects a person's vocation in life and role in the community, are made publicly, and before a priest or public official. A Christian who makes an oath to God is responsible for it, not to the peril of his soul, but as a sin if he breaks it.
Certain sects of Western Christianity, amongst them the Religious Society of Friends and the Mennonites, object to the taking of both oaths and affirmations, basing their objections upon a commandment given in the Sermon on the Mount, and regard all promises to be witnessed by God.
Before the life, spirits of all people has promised Allah that they would accept Tawheed, if they are sent to the Earth. Ones who accept that will go to Jannah and ones who break their promises will go to Jahannam. However, Allah forbids all people who turns back to Islam, which they are born with it. In An-Nahl 91, Allah forbids Muslims to break their promises after they have confirmed them. All promises are regarded as having Allah as their witness and guarantor. In a Hadith, Muhammed states that a Muslim who made a promise and then saw a better thing to do, should do the better thing and then make an act of atonement for breaking the promise."
After reading this, I got an idea that when people lived under religious rules every aspect of life was covered by religion and everybody was supposed to promise to follow the religion. Therefore they didn't have to make other promises.