The preposition “para” is used in this sentence to express the purpose or goal of the action. In other words, the speaker is saying that they were foolish enough to have the purpose or goal of believing the statement.
The preposition “por” could also be used in this sentence, but it would have a different meaning. “Por” would express the reason or cause for the action. In other words, the speaker could be saying that they were foolish enough to believe the statement because they were gullible or naive.
This is interesting. I had never thought of using “por” in this context; in my mind, with “por” I get the meaing “The fact that I believed it meant I was stupid enough”.
Does your reasoning also apply to a sentence such as “Fui lo suficientemente mayor para poder votar”?
I think you’re wrong about “para” here. Under most circumstances, it doesn’t make sense to talk about having the purpose or goal for believing something. Most of the time, people would use it like “I was stupid enough to [be able to] believe it” when saying this, and purposes and goals don’t come into play at all here. “Para creerlo” marks a consequence of the speaker’s stupidity, which is different from a purpose or goal.
With “por,” the meaning is more like “I was stupid enough because I believed it.” The way you explained it is bad. “Por creerlo” isn’t marking a cause of stupidity, it’s marking evidence that the person was stupid.
The English sentence doesn’t look like it has to be a causal clause. There are probably cases where it isn’t meant that way. But I think this one is, by default, usually meant that way. People blame stupidity for lots of things, it makes sense to suppose the person meant his own was the reason why he “believed it.”
Maybe that was what Aricus meant in his comment, now that I think of it? I’ll know when I get a response.