Cultist's invitation is a paper note in Fallout 76.
Location
The note can be found on a dead member of the Cult of the Mothman, during the Dead Cultists random encounter.
Transcript
Sister Acolyte,
Let's go to the secret place of worship at midnight and pray to the Holy Mothman.
I believe with this show of devotion, and your great beauty, He will be moved to bless us with a visitation.
The mere thought fills my soul with joy! Will you join me?