On this webpage it states that "The Simple Present can also indicate the speaker believes that a fact was true before, is true now, and will be true in the future. It is not important if the speaker is correct about the fact." The examples are straightforward, but I am still a little bit confused about tense. Which tense of simple present and simple past should I use in the following examples?
When he was a kid he thought that the earth was/is flat. Now he knows it's round.
In the 5th century people believed that the earth was/is flat.
I think in both examples is instead of was should be adopted. The reason is that the subjects (the kid and people in the 5th century) considered the proposition to be universally true, and according to the foregoing rule it doesn't matter whether they were correct.
Am I understanding the grammar in the correct way? Many thanks for your help :-)