land-of-brains-and-chocolate:

“sacred” was first used to describe symbiosis in general by eddie in planet of the symbiotes… even though, in that story, all the klyntar the venom symbiote knew were acting like parasites and it was the only one who cared for its host.

eddie’s reaction to finding this out is not “you lied to me” but “you never told me this”, which means that the symbiote likely avoided the topic of its home and species altogether. so it must have, at some point, described their specific bond and the way it feels about it as “sacred”.

it’s also possible that eddie came up with it entirely on his own. either way, it’s a pretty loaded word for someone with a catholic background, and it’s really something for eddie to draw the conclusion that the way he experiences their relationship is so profound that there must be something inherent about symbiosis that makes it so

(and to find that there isn’t)