Where does the sentence, “Have no fear of them, nor be troubled” come from?

Peter is quoting here from Isaiah 8:12–13. King Ahaz and his people were under threat from a cruel Assyrian army. In that context Isaiah spoke words of promise and admonition, but experienced opposition from conspirators who seemed very eager to destroy him on account of his prophetic words. So the Lord warned the prophet not to walk in the way of this people with this charge: Do not call conspiracy all that this people calls conspiracy, and do not fear what they fear, nor be in dread. On the contrary, the Lord of hosts, him you shall honor as holy. Let him be your fear and let him be your dread. Here, fear of what scares people is set over against fear of God, for God is mightier than the Assyrians.

Now in a similar situation, with the persecutions brewing, Peter warns his brethren to not fear or be troubled by the intimidation of the opposition.¹There is a slight but significant change in Peter’s rendering of Isaiah’s words. Isaiah had written, Do not fear what they fear, with the allusion clearly being to a third party, namely, the Assyrians. But Peter writes, “Have no fear of them.” The plural pronoun refers to the people making things hard for Peter’s readers (as opposed to some distant foreigners). It is these locals whom the Christians must not fear.