As seen throughout the book of Hebrews, sacrifices were extremely important under the Old Covenant. Under the New Covenant, God desires the praise and thanksgiving of His people rather than offerings of animals or grain. Since New Testament believers are priests (1 Peter 2:5,9), they have offerings of praise and thanks to God (Romans 12:1).
Commentary from the MacArthur study Bible, notes for Hebrews 13:15.
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