Heaven's Gate members believed that in the future Earth was going to be recycled and in order to escape this they needed to leave Earth via spaceships. They believed that these extraterrestials in spaceships would come and pick up their souls and bring them to the Next Level, also known as "The Evolutionary Level Above Human" or "The Kingdom of Heaven," and they believed this place to be a real physical place.
In order to reach this Next Level, Heaven's Gate practitioners believed they had to detach themselves from human characteristics. This includes their family and friends, all of their money and other things they owned, sex, even their appearance (They wore similar clothing and got similar haircuts). Some 7 of the members even went as far to go to Mexico in order to castrate themselves and deny themselves of human urges.
Heaven's Gate members also believed that malevolent aliens were sent to disrupt all other religions and stop them from reaching the Next Level. Their core beliefs didn't change much, but later on they would have small changes like altering how one gets to the Next Level. The group also began to adopt "New Age" ideas that formed in the religious and spiritual fervor of the 1970s. One of the biggest beliefs were walk-ins, where a body becomes occupied by some separate entity. Nettles and Applewhite had considered themselves the extraterrestial equivalent of "walk-ins", claiming that they become extraterrestials that were just inhabiting the human bodies.