The authority to speak for Jesus comes from being with Jesus. You can't do the one without the other.
Jesus called His disciples to be with Him and then He sent them out to preach and exercise authority over demons (Mark 3.14-15). Can you imagine taking on such a monumental task - declaring and demonstrating the Kingdom of God - without first being with the King and hearing His heartbeat, knowing what's on His mind, seeing where He's at work and where He's planning on working next.
The reason why the Kingdom works in this manner is because of the secret nature of it. So much of what goes on in the Kingdom is conspiratorially quiet, secretive, unseen - like plants sprouting and vines producing fruit and the smallest of seeds growing into some of the largest plants in a garden. Sure, there are outward Kingdom manifestations. The exercise of that kind of authority requires a connectedness to the King too. But much more goes on in the quiet, where Jesus does His best work. And that certainly demands an intimacy with the King. Be with Him in secret so He can use you to work on His behalf in secret.