The Greek adjective aspilos means spotless, unstained, or without blemish — literally having no spot (spilos = spot, stain). It is applied to Christ as the spotless Lamb whose sacrifice atones for sin, and to the believer's calling to live in moral purity before God.
Aspilos reaches back to the Old Testament sacrificial system where only unblemished animals could be offered to God (Leviticus 1:3). Christ fulfilled this requirement perfectly — Peter declares Him 'a lamb without blemish or defect' (1 Peter 1:19). But the language is turned toward the believer: we are to keep ourselves 'spotless' (James 1:27) and be found 'spotless, blameless and at peace with him' (2 Peter 3:14). What Christ is by nature, we are called to become by grace — a transformation made possible only through His spotless sacrifice.