A lot vs. Alot: 9 Grammatical Pitfalls
from Old English eornoste (adj.) "zealous," or from Old English noun eornost "seriousness, serious intent" (surviving only in the phrase in earnest), from Proto-Germanic *ern "vigor, briskness" (cf. Old Saxon ernust, Old High German arnust "seriousness, firmness, struggle," German Ernst "seriousness;" Gothic arniba "safely, securely;" Old Norse ern "able, vigorous," jarna "fight, combat"). The proper name Ernest (literally "resolute") is from the same root. Related: Earnestly; earnestness.
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.