The phrase "set apart the Lord Christ ... as holy" is a metaphor for acknowledging Christ's holiness. Here "hearts" is a metonym for the "inner person." AT: "acknowledge within yourselves that the Lord Christ is holy" or "honor the Lord Christ as holy within yourselves" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-metonymy]])