Why was Emerson's speech "The Divinity School Address" so controversial?

Emerson's "Divinity School Address" was hugely controversial because it presented a vision of Christianity too unorthodox even for the most radical of Unitarians. For one thing, Emerson openly criticizes the role of religious authorities in instructing the faithful. As a radical individualist, Emerson believes he can only appropriate the truth for himself; it's not enough to go on someone else's word, however wise or learned or devout they may be. Whatever someone says in relation to religion—or anything else, for that matter—the truth can only be found in me. Otherwise, it must be rejected.
One can only imagine the reaction of the audience to this particular passage. Here is Emerson, speaking at one of the country's foremost schools of divinity, essentially saying that there is no role for institutional authority or instruction. In saying this, he's undermining the whole rationale of Harvard Divinity School and similar institutions. No wonder he wasn't invited back to Harvard for more than thirty years!
Emerson goes on to criticize the way in which Christianity as it has developed historically, has stifled man's moral nature, which to Emerson is the ultimate law by which we should live. This unfortunate situation has been achieved by an undue focus on the past, which is worshipped and venerated at the expense of the present. Furthermore, Christianity's emphasis on the historical Jesus has tended to obscure Christ's true spiritual message, which for Emerson is what's really important.