Fausset's Bible Dictionary
2 Corinthians 1:20; 2 Corinthians 1:22; 2 Corinthians 5:5; Ephesians 1:13-14. Money given by a purchaser as a pledge for the full payment of the sum promised. The Holy Spirit is to the believer the first installment to assure him that his full inheritance as a son of God shall follow hereafter; the token of the fulfillment of "all the promises." Hence, the Spirit is called "the Holy Spirit of promise," "the first fruits of the Spirit" (Romans 8:23), i.e., we have the Spirit Himself as the first fruits of our full redemption. Hebrew arabon , brought by the Phoenicians to Greece and Rome, Latin arrhabo .
The payment of an earnest or deposit bound both seller and purchaser to carry out the contract (i.e. a guarantee, a down payment). This partial payment implies the identity in kind of the deposit with the future full payment; but a "pledge" may be of a quite different kind (Genesis 38:17-18). "Earnest" implies, besides the security of the believer's future inheritance, its identity in kind, though not in degree, with his present possessed enjoyment of the spirit. Heaven perfected will continue heaven already begun in part (Revelation 22:11 ff).
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Fausset, Andrew R. Entry for 'Earnest'. Fausset's Bible Dictionary. https://www.studylight.org/dictionaries/eng/fbd/e/earnest.html. 1949.