Because God's Word is pure (v. 5), adding to it is a serious offense — it corrupts the purity of what God has said and places human words on the same level as God's. This warning resonates throughout Scripture (Deuteronomy 4:2; Revelation 22:18–19) and guards against false teaching, adding traditions to Scripture, or claiming God said things He did not.
Memory phrase: Never add to God's Word.
Talk about it: Why is it wrong to say God said something He did not say?
Prayer: God, help me say only what You have said.