The phrase "set apart the Lord Christ ... as holy" means to acknowledge Christ's holiness. This action is referred to as if it were putting something in a place of honor. (See: [[rc://en/ta/man/translate/figs-metaphor]])
"Hearts is a metonym for "thoughts and intentions." AT: "set apart the Lord Christ as holy in all of your thoughts and the way you act" (See: [[rc://en/ta/man/translate/figs-metonymy]])