I don't think that there is any abuse in the act of teaching them religion...but being overly pushy and refusing to let them think for themselves, at a certain age, as to the direction of the religious experience (if they care to pursue religion at all), is abuse. Forcing religion upon someone who clearly doesn't believe/agree is a form of abuse - psychological and mental.

Depends on how far they go with it. If they exclude everything else but their religious teachings, I think that's doing a huge disservice to the kid's mental health.

Teaching them about religions, no. But forcing them to believe one way, yes. Religion should never be demanded or forced onto anyone. I do feel understanding where the religious minded people are coming from helps navigate through life because you cannot avoid them and making waves by butting heads only causes problems for everyone involved, especially in work or school settings, often in families as well. But children shouldn't be forced into any one way of belief or punished for not believing. I'd consider that abuse in both mental and emotional ways.

Yes. No teaching until at least aged 7. And then teach all beliefs and disbeliefs!

Kind of. Children want to please, so they tend to assimilate world views to satisfy others. It’s innocent, and taken advantage of. Unless all faiths are covered, then it’s really just indoctrination.