Your very first interaction with the Brotherhood of Steel way back in Fallout involved them sending you on a suicide mission and laughing their asses off the moment you left. They are legitimately surprised when you actually return. Yeah. Real heroes.
From the Fallout Wiki's Brotherhood of Steel page:
"In 2162, the Vault-Dweller arrived at the Lost Hills bunker, seeking entry into the Brotherhood. His request was not taken seriously and he was tasked with retrieving the records of Sergeant Dennis Allen's expedition from the hazardous ruins of the Glow, a fool's errand commonly used to deter would-be Brotherhood members. To the Brotherhood's surprise, not only did the Vault-Dweller return from the West-Tek facility alive, but he had also succeeded in his fool's errand and recovered a holodisk recording of Allen's doomed expedition. As a result, the Vault-Dweller became the first outsider in decades to be accepted into the Brotherhood of Steel, despite the protests of a few members."
From the wiki's Glow page:
"The Brotherhood of Steel used to send unwanted applicants to the Glow on a "fool's quest", referring to it by the moniker the "Ancient Order", just to laugh about it later when they ended up dead because of the high amount of radiation in the area. They were quite surprised when one known as the Vault Dweller actually made it back, bringing with them a holodisk that recorded the fate of Allen's expedition, and earning them a place in the Brotherhood.
It is important to note that the Glow, true to its name, is one of the most highly irradiated locations in the Fallout universe. A few moments' exposure is enough to give a lethal dose of rads. Use of Rad-X to boost Radiation Resistance to 100% is widely considered absolutely necessary to survive a trip to the Glow."
From the Fallout Wiki's Brotherhood of Steel page:
"In 2162, the Vault-Dweller arrived at the Lost Hills bunker, seeking entry into the Brotherhood. His request was not taken seriously and he was tasked with retrieving the records of Sergeant Dennis Allen's expedition from the hazardous ruins of the Glow, a fool's errand commonly used to deter would-be Brotherhood members. To the Brotherhood's surprise, not only did the Vault-Dweller return from the West-Tek facility alive, but he had also succeeded in his fool's errand and recovered a holodisk recording of Allen's doomed expedition. As a result, the Vault-Dweller became the first outsider in decades to be accepted into the Brotherhood of Steel, despite the protests of a few members."
From the wiki's Glow page:
"The Brotherhood of Steel used to send unwanted applicants to the Glow on a "fool's quest", referring to it by the moniker the "Ancient Order", just to laugh about it later when they ended up dead because of the high amount of radiation in the area. They were quite surprised when one known as the Vault Dweller actually made it back, bringing with them a holodisk that recorded the fate of Allen's expedition, and earning them a place in the Brotherhood.
It is important to note that the Glow, true to its name, is one of the most highly irradiated locations in the Fallout universe. A few moments' exposure is enough to give a lethal dose of rads. Use of Rad-X to boost Radiation Resistance to 100% is widely considered absolutely necessary to survive a trip to the Glow."