You wrote: “Second, Spinoza’s proposal is that we believe a proposition upon it simply entering our mind, completely independently of the source of the proposition.”
So… off the top of my head, I’m assuming that the brain receives a “proposition” (someone telling us something) the same way it receives an “experience” (anything happening we can perceive in the world.) Both are simply sensory data being interpreted.
In that case, the easiest answer seems most likely, we are wired to believe the reality of what we perceive… “That is a hole, don’t walk into it.” “That is a pointy stick coming at you. Duck.” “That is food, eat it.”
We fundamentally survive by believing the world around us is real, then test what our senses tell us to see if true. “Eat it, maybe get sick, maybe not.” “Touch the pointy stick, verify it would hurt us.” etc.
Same thing for a proposition… it is just the outside reality informing our senses, but abstracted into the realm of language and ideas… still the same process in our brains. Accept that it is real (both the voice/sound and the words/meaning) and then test it through experience to validate or invalidate our perception.
I’m sure I’m missing something, but start with the simplest idea first.