Learn Spanish (Latin America): No estamos de acuerdo con esa idea.

In Spanish, there isn't a single everyday verb that means "to agree." Instead, you use the phrase estar de acuerdo, which literally translates to "to be of agreement." Since the subject is "we," we conjugate estar to estamos.

Does estar de acuerdo always take the preposition con?

Yes, when you want to say you agree with a person, a statement, or an idea, you use con. The full combination you should memorize as a chunk is estar de acuerdo con (to agree with).

Since an opinion can be a long-lasting belief, why do we use estar instead of ser here?

Even though an opinion might feel like a permanent trait, estar de acuerdo is a fixed phrase in Spanish. You must always use estar for agreeing or disagreeing, regardless of how strongly or permanently you hold that view.

Why is it esa idea and not ese idea?

The word idea is a feminine noun in Spanish. Because adjectives must match the gender of the noun they describe, we use the feminine word for "that" (esa). If the noun were masculine, like plan (plan), you would use ese, as in ese plan.