Truthful sounds easy but is very hard. Everyone has a bias. On the Voyager record we decided to put our best foot forward and avoided depicting war, poverty or disease. For various reasons I stand by this position and would probably make the same decision today. But many people criticized us for telling a half-truth, if not an outright falsification about what Earth is like.
Outright deception is easy to forbid.
But the decision about which information to include also excludes. That is the hardest choice, best made in good faith by one group and judged in good faith by another. Nondenominational is a little easier, but only a little. The message can't come from Mecca, the Vatican, or the US Congress. It must avoid endorsing any particular religion, philosophy or ideology.