When I try to download Bitcoin Core on MacOS it says "Apple cannot check it for malicious software"? Is it safe to download?
As mentioned by Andrew, this is due to macOS' enforcement of notarization requirements.
This can be bypassed by right clicking on the application, then selecting Open - A dialog will popup warning you that it is not verified, but this one will have an Open button.
Once you open the application through the above process once, a regular double click will work on it in the future.
You may need to go through this process again if you update to a newer version of Bitcoin Core.