There's ways to state your opinion and attempt to persuade people that it's right without using 1st person. The person reading the essay knows that the opinion is your own, so you don't need to tell them that 'I think blah blah'. You'd start off your essay by stating your opinion as fact, and then you'd explain to the reader why it is a fact/why my hypothesis is true. Then in your actual body paragraphs, you'd break down each of those reasons/justifications.