The Spirit is the earnest of the believer's destined inheritance (2 Corinthians 1:22; 5:5; Ephesians 1:14). The word thus rendered is the same as that rendered "pledge" in Genesis 38:17-20; "indeed, the Hebrew word has simply passed into the Greek and Latin languages, probably through commercial dealings with the Phoenicians, the great trading people of ancient days. Originally it meant no more than a pledge; but in common usage it came to denote that particular kind of pledge which is a part of the full price of an article paid in advance; and as it is joined with the figure of a seal when applied to the Spirit, it seems to be used by Paul in this specific sense." The Spirit's gracious presence and working in believers is a foretaste to them of the blessedness of heaven. God is graciously pleased to give not only pledges but foretastes of future blessedness.
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Easton, Matthew George. "Entry for 'Earnest'". "Easton's Bible Dictionary".