If you don't want to know the circumstances surrounding Monday's entry in this slideshow, then don't read any further...
[spoiler]Monday's quote:
Ep #600 (1968) - Julia - 'Barnabas, if it's, if it's Nicholas, he MUST know about the experiment.'
From "Robservations 8/12/02 #600/601 - A Chilling Summons"
]Barnabas surmises that Cassandra had to have told Nicholas everything about the past--if he did erase Maggie's memory, he did it knowing he was helping him, and there's only one reason for that--Adam--Blair knows if Barnabas is in danger, so is Adam. Julia says if it is Nicholas, he must know about the experiment, but they need proof.
While not referencing the entire quote, the Robservations references the quote quite well...
And this is the third quote from this ep. The first (Prof. Stokes - 'No, you MUST tell us who lives in that house) was back on November 23rd of 2020, and the second (Prof. Stokes - 'There MUST be a connection. If there were only some way of making contact') was back on April 22nd of 2021.