The Spirit is the earnest of the believer’s destined inheritance (2 Cor. 1:22; 5:5; Eph. 1:14). The word thus rendered is the same as that rendered “pledge” in Gen. 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.
Earnest
An earnest is a first payment that promises the rest will follow, and Paul uses the word twice about the Spirit. In 2 Corinthians 1:22 God “hath also sealed us, and given the earnest of the Spirit in our hearts.” In chapter 5 Paul says the same, in the middle of a passage about the hope of a new body. So the Spirit now is a pledge of more to come. Psalm 86:17 is a related plea: “Shew me a token for good.” Read 2 Corinthians 5:1-5 for the fuller thought.
3 verses
Psalms
Shew me a token for good; that they which hate me may see it, and be ashamed: because thou, LORD, hast holpen me, and comforted me.
2 Corinthians
Who hath also sealed us, and given the earnest of the Spirit in our hearts.
Now he that hath wrought us for the selfsame thing is God, who also hath given unto us the earnest of the Spirit.
From Easton's Bible Dictionary
Matthew George Easton, 1897