Technical Tip: CA Certificate is not trusted on macOS, even though it was imported correctly

When importing a CA certificate in MacOS, it will go into something called the Keychain.
This can be accessed by searching for 'Keychain Access' in Spotlight, or by opening a certificate file. 

Even after importing the CA certificate, the Keychain will not implicitly trust the certificates it has installed. 

It is possible to know if it is not trusted as it has a red X through it, and the message 'This root certificate is not trusted’ appears.
It is necessary to 'right-click' the cert, select ‘Get Info’ then select ‘Always Trust'.