Grace fundamentally means “that which causes joy,” a shade of meaning we may still discern when we speak of a graceful action or the social graces. It comes to mean “favor,” “kindness,” and then especially God’s kindness to man for providing for his spiritual needs in Christ. Thence it comes to signify what is due to grace, namely, God’s good gifts to men, and finally the attitude of thankfulness which all this awakes within the Christian. As used in greetings it is the free gift of God that is meant, but the word necessarily evokes memories of the free gift on Calvary.
Leon Morris, First and Second Epistles to the Thessalonians, 49