1Peter:3:10: For he that will love life, and see good days, let him refrain his tongue from evil, and his lips that they speak no guile: 1Peter:3:11: Let him eschew evil, and do good; let him seek peace, and ensue it.
"Rest in peace" is sort of a non-denomonational way of saying that whatever one's personal belief may it result in peace for the deceased if appropriate and also those left behind.
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"...only the shadow knows"