This from the 

Anatta-lakkhana Sutta: The Discourse on the Not-self Characteristic

is a declaration by an authority to some persons who accepted his words at face value. It is not hard evidence with proofs. In the Vedic setting however, at the time of Buddha it was the convention to accept an authority at his word so that whatever he said was taken at face value.

It begins with declaring that

form is not-self

but there is no proof

Hence that means that we have to accept that person's word as proof even though we have not experienced the proof.

It is an exercise in confidence, more or less. At the end of the day, we are still looking to experience the proof.

Suppose for argument sake, I declare that 

form is self

and a group of Michael Beloved fanatics accept that, then what?