This instruction completes Peter’s allusion to Isaiah 8:12–13. When the Lord commanded the prophet not to fear what people fear, he added an instruction that Isaiah was to do instead: The Lord of hosts, him you shall honor as holy.
Peter’s readers are to do the same.
15 but in your hearts honor Christ the Lord as holy, always being prepared to make a defense to anyone who asks you for a reason for the hope that is in you; yet do it with gentleness and respect,