Learn Finnish: Pidän häntä tärkeänä, joten kutsumme häntä kunniajäseneksi. - I consider him important, so we call him an honorary member.

This is dictated by the verb kutsua. When kutsua means to call someone a name or title, it requires the assigned role to be in the translative case (-ksi). This is a strict rection rule for verbs of naming, electing, and appointing, such as nimittää and valita.

Why do we use the essive case (tärkeänä) in the first part of the sentence instead of the translative?

The verb pitää translates to consider or regard as when used with a partitive object and an essive complement (-na/-nä). While the translative implies a change or a newly assigned title, the essive expresses a subjective opinion about someone's current, existing state. Therefore, I consider him important becomes pidän häntä tärkeänä.

Both verbs act on him, so why is häntä in the partitive case rather than the accusative (hänet)?

Verbs expressing attitudes, opinions, or continuous states almost always take a partitive object. Both pitää (to consider) and kutsua (to call someone something) fall into this category. The action of considering or calling someone by a title does not have a discrete physical end point, so the object remains in the partitive.

Kutsua often means to invite. How can I easily tell that it means to call in this sentence?