I am not sure if the audience of Christ would have accepted him if he came with a letter of authority. Such were the ways of the time: Paul himself noted that he was trained by Gamaliel. What he was saying would have not been accepted regardless.
The issue for this time is that no authority is accepted, and the plain truth is denied. For such, use parables and rhetoric.
20 One day, as Jesus was teaching the people in the temple and preaching the gospel, the chief priests and the scribes with the elders came up 2 and said to him, “Tell us by what authority you do these things, or who it is that gave you this authority.” 3 He answered them, “I also will ask you a question. Now tell me, 4 was the baptism of John from heaven or from man?” 5 And they discussed it with one another, saying, “If we say, ‘From heaven’, he will say, ‘Why did you not believe him?’ 6 But if we say, ‘From man’, all the people will stone us to death, for they are convinced that John was a prophet.” 7 So they answered that they did not know where it came from. 8 And Jesus said to them, “Neither will I tell you by what authority I do these things.”
English Standard Version
The current elite will not accept the authority of the word of God, yet defend their right to micromanage those of the faith. The measure they use will be used against them. Use them as an example of what not to be.