En ce temps là, je pensais que c’était/qu’il était un idiot.

The answer is

En ce temps là, je pensais qu’il était un idiot.

It’s the translation of “back then, ….” Because now you think that he’s not. If you think he’s actually an idiot you say

Depuis ce temps là je sais qu’il est un idiot.

And I think in informal french you can use the second sentence in the two cases.

The right translation of “Back then, I thought that he was an idiot” is

En ce temps là/ à cette époque là, je pensais que c’était un idiot

You can’t say Je pensais que c’est un idiot, because you alreay used the verb penser in the past time (Imparfait)

If you wanted to use the present, you should have said

A cette époque là, je pense que c’est un idiot

The same thing for the sentence “En Mars, je vais chez lui”. It’s the present tense, but you know that is something happening in the past (in March)

But why the present is used for past events, you could ask ?

Well, it’s what we call in french Le présent de narration, and it is used to relate past events with a more vivid appearance, like if they were happening now. It is, for example, frenquently used in novels :

Ce matin-là, j’arrive en retard.

This présent de narration has the same value of a past tense.