EARNEST (Gr. arrabōn). A legal term in English law denoting the payment of a sum of money to make a contract binding, guaranteeing a further payment to fulfill the contract. Thus the significance of the apostle Paul’s use of this word in regard to the Holy Spirit in three passages may be understood (2Cor.1.22; 2Cor.5.5; Eph.1.14). The NIV renders “seal” in the first passage, “deposit” in the two others. The Holy Spirit’s gift to believers is the assurance that their redemption will be fully carried out.
In a derived sense an ἀρραβών, G775, in a commercial transaction came to be a down payment, as in the modern hire-purchase system (some good examples are given in MM). Both meanings, pledge and first installment, are involved in each of the three NT uses of the word. 2 Corinthians 1:22 speaks of God’s gift of the Holy Spirit as the pledge and foretaste of what the Christian will enjoy later (significantly the word “seal” is also used in the context). 2 Corinthians 5:5 similarly says that the Holy Spirit is the earnest of that fullness of life which the Christian will enjoy after the dissolution of his earthly “tent.” The Holy Spirit of promise is the earnest of the inheritance (Eph 1:14) which the Christian will finally receive. The earnest, as Behm (in TWNT) puts it, “always implies an act which engages to something bigger”; it is a pledge or deposit guaranteeing that a larger payment will be made.
International Standard Bible Encyclopedia (1915)
Found three times in the New Testament: The "earnest of our inheritance" (Eph 1:14); "the earnest of the Spirit" (2Co 1:22; 5:5).
It has an equivalent in Hebrew `erabhon (found in Ge 38:17,18,20), in Latin arrabo, French arrhes and the Old English arles. The term is mercantile and comes originally from the Phoenicians. Its general meaning is that of a pledge or token given as the assurance of the fulfillment of a bargain or promise. It also carries with it the idea of forfeit, such as is now common in land deals, only from the obverse side. In other words, the one promising to convey property, wages or blessing binds the promise with an advance gift or pledge partaking of the quality of the benefit to be bestowed. If the agreement be about wages, then a part of the wages is advanced; if it be about land, then a clod given to the purchaser or beneficiary may stand as the pledge of final and complete conveyance of the property.
Figurative: In the spiritual sense, as used in the passages above named, the reference is to the work of the Spirit of God in our hearts being a token and pledge of a perfect redemption and a heavenly inheritance. There is more than the idea of security in the word as used, for it clearly implies the continuity and identity of the blessing.