Paul is borrowing from the Greek world. Hail
(chaire) was a common Greek salutation. However, Paul modified this and spoke almost the same sound but a different word with a different meaning when using the word grace
(charis).1 Grace is a key theological concept for Paul. It says about God’s new covenant people that their status is ultimately dependent on God’s own unmerited intervention on their behalf.2
The richness of this meaning is expressed well by John Piper: At the beginning of his letters Paul has in mind that the letter itself is a channel of God’s grace to the readers. Grace is about to flow from God through Paul’s writing to the Christians. So he says, grace to you. That is, grace is now active and is about to flow from God through my inspired writing to you as you read – grace be to you…thus we learn that grace is ready to flow to us every time we take up the inspired scriptures to read them.
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2 To the saints and faithful brothers in Christ at Colossae: Grace to you and peace from God our Father.