How can I delete non-Christian holidays from the us Holiday calendar?

A calendar that you have subscribed to can not be customized. If you wish to not see certain entries, you will need to seek a different subscription, or unsubscribe from the existing and create your own events.

You can't modify the default holiday calendar.

Unsubscribe from it and subscribe to something that gives you what you want instead. That's what I had to do to get rid of all the x-tian holidays.

Use iCloud calendar subscriptions - Apple Support

This still makes me laugh. Everyone flipped out over Easter being removed and now are all upset that they have to see non-Christian holidays.