In chapter 23 of The Scarlet Letter we find Reverend Arthur Dimmesdale at the height of his reputation. He just finished giving a sermon as part of the Election Day holiday celebration for which he had been preparing for quite some time. In fact, he was insistent to Hester that whatever preparation they ever plan to make together had to happen after this speech is given.
During this speech, Dimmesdale spoke with utmost charisma, filling the atmosphere with a supernatural air of spirituality and morality that is described as
the high spell that had transported [the flock] into the region of another’s mind
The effect is such that, after the speech is over, the villagers feel the need to take in fresh air, as if the words spoken by Dimmesdale had seared within them. What the author calls the villagers' "united testimony" refers to the agreement to which they all reached regarding Dimmesdale's words. The agreement is that the speech is the finest ever spoken in the village.
According to their united testimony, never had man spoken in so wise, so high, and so holy a spirit, as he that spake this day; nor had inspiration ever breathed through mortal lips more evidently than it did through his.
Therefore the townsmen agree that Rev. Dimmesdale's words solidify him as one (if not the) holiest men to ever be known in the settlement.