I’ve just been told by “someone” that when we visit Zion National Park we must take a bus up to the Temple of Sinawava. Is this true?
During the main season at the park (spring to fall, essentially), private vehicles are banned on the main park roads. There would simply be too many of them.
You can park in various parking areas near the park gates, and there is a free bus that will transport you around the park. It's quite convenient and runs fairly frequently. I thought it would be really inconvenient, but it isn't.
If you absolutely must drive your own vehicle on the roads, you can usually do so in the winter. Check the park's website for details on when private vehicles are allowed.