Grace (Latin gratia; Greek charis) is a gift. A gift is something gratuitously bestowed. The giver is free to offer the gift; the receiver is free to accept or to reject it. If either the giver or the receiver is compelled, it is not a true gift, for a compulsory exchange implies a duty, which is a matter of justice, rather than of mercy. In Book II of his treatise De Peccatorum meritis et remissione (On the Merits and Remission of Sins), St. Augustine of Hippo (354-430) discusses issues relating to the gratuity of the various graces God offers to us.
As to how gratuity relates to a person’s salvation, one might wonder “Why, however, He helps one man, but not another; or why one man so much, and another so much; or why one man in one way, and another in another.” The answer to this, St. Augustine notes, is that “He reserves to Himself according to the method of His own most secret justice, and to the excellency of His power.” (II, 6)
In the Parable of the Laborers in the Vineyard (Matt 20:1-16), Christ illustrates this principle by telling us of certain vineyard workers who each agreed to accept whatever wage the foreman would give them, but who, at the end of the day, complained because those who had worked less hours received the same wage. But, the bottom line is this, said the foreman: “Is it not lawful for me to do what I will with mine own?” (Matt 20:15).
St. Augustine elaborates: “As to the reason why He wills to convert some, and to punish others for turning away,—although nobody can justly censure the merciful One in conferring His blessing, nor can any man justly find fault with the truthful One in awarding His punishment (as no one could justly blame Him, in the parable of the labourers, for assigning to some their stipulated hire, and to others unstipulated largess), yet, after all, the purpose of His more hidden judgment is in His own power. So far as it has been given us, let us have wisdom, and let us understand that the good Lord God sometimes withholds even from His saints either the certain knowledge or the triumphant joy of a good work, just in order that they may discover that it is not from themselves, but from Him that they receive the light which illuminates their darkness, and the sweet grace which causes their land to yield her fruit.” (II, 32)
Hence, the admonition of St. Paul: “What hast thou that thou didst not receive? Now, if thou didst receive it, why dost thou glory, as if thou hadst not received it?” (1 Cor 4:7) (II, 28)
Quotations from A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church, Vol. V, ed. Philip Schaff (Edinburgh: T&T Clark, 1886).