Legal Question in Constitutional Law in Ohio

The message you describe is protected, but the method is not. The government cannot restrict speech based upon its content, but it can place reasonable restrictions on the time, place and manner of speech. If the megaphone was unreasonably loud (especially if this was in a residential area late at night) and/or if it was used in a way that harassed passersby, the preacher could be cited or arrested for violating noise ordinances or for disorderly conduct. There may be other grounds for arrest or citation as well.

Note that the legality of the preacher's actions have nothing to do with what he is saying and everything to do with how he is saying it. The government cannot restrict religious speech differently from other speech. That fact works both ways, though. Religious speech is generally not protected if non-religious speech under the same circumstances also wouldn't be protected.