Is it forbidden and sinful to take religious pictures and natural scenery from other people's pages on social media without their knowledge?

It is permissible to copy images if their owners do not object. If they prohibit commercial use but not personal use, then it is permissible to copy them for personal use only.

However, if they absolutely prohibit copying (either explicitly or implicitly), then it is not permissible to copy them without their permission, and copying would then be forbidden.

If the stance on copying is unknown, the default is permissibility, provided that the images themselves are originally permissible.