When God tells you to be something, He is the one who is going to make you into that something.
“Be Holy as I am Holy…” (Leviticus 20:7, 1 Peter 1:16)
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“By God’s will we have been made holy through the offering of the body of Jesus…” (Hebrews 10:10)
“Therefore you shall be perfect, just as your Father in heaven is perfect.” (Matthew 5:48)
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“For by one offering He has perfected forever those who are being sanctified.” (Hebrews 10:14)
The reason it says “be” instead of “become” is because it isn’t a slow transformation you make through a lifetime of good behavior. It is an immediate transformation He makes through His blood shed and His body risen.