The greatness of the message comes from the greatness of who sent it, so the message from Allah is very great. The greatness of the messenger also comes from who sent the messenger, so the messenger of Allah is very great. When Allah (SWT) Says (O, Messenger, convey what has been sent to you from your Lord, and if you don’t, you will be then like you did not convey His message) (5:67), thus means that thus is the greatest message from Allah to be conveyed by His messenger, which is equal to the whole message of Islam and not conveying it is equal to not conveying the whole message of Islam.
Wassalam.