Ephesians 1:14 calls the Holy Spirit “the earnest of our inheritance until the redemption of the purchased possession.” The word “earnest” (ἀρραβὼν) was a common term referring to an advance or down payment. Here are a couple first-century examples.
The document to the left (P.Fay.91) is a first-century (A.D. 99) employment contract of a woman named Thenetkoueis to serve for the season in an oil-press belonging to Lucius Bellenus Gemellus at a daily wage, the exact amount of which is not stated, but of which she receives an advance of 16 drachmae.
The word for “advance” (used on line 14) is the same word Paul uses in Ephesians 1:14 in reference to the Holy Spirit. Just as an advance on a paycheck gives employees what they presently need and proves that more payment will come, the Holy Spirit has been given to us both to satisfy our present need and to prove that God will one day completely reward us.
The term “earnest” (ἀρραβὼν) was used in reference not only to advances on employee paychecks but also to down payments of any sort. The term could even be used to refer to a down payment payed to one’s exterminator. A first-century letter (P.Oxy. 299) reads,
“Horus to his esteemed Apion greeting. Regarding Lampon the mouse-catcher I paid him for you as earnest money 8 drachmae in order that he may catch the mice while they are with young. Please send me the money. I have also lent Dionysius, the chief man of Nemerae, 8 drachmae, and he has not repaid them, to which I call your attention. Good-bye.” (The Oxyrhynchus Papyri, vol 2, pp. 300-301).
The Holy Spirit is the earnest of our inheritance from God. Like an advance on a paycheck, the Holy Spirit gives us what we need for today. Like a down payment, the Holy Spirit proves that we will receive our full reward.