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originally, "earnest-money" deposited by the purchaser and forfeited if the purchase was not completed, was probably a Phoenician word, introduced into Greece. In general usage it came to denote "a pledge" or "earnest" of any sort; in the NT it is used only of that which is assured by God to believers; it is said of the Holy Spirit as the Divine "pledge" of all their future blessedness, 2 Corinthians 1:22; 5:5; in Ephesians 1:14; 4:30 particularly of their eternal inheritance.
2 Corinthians 1:21 Now it is God who makes both us and you stand firm in Christ. He anointed us, 22 set his seal of ownership on us, and put his Spirit in our hearts as a deposit, guaranteeing what is to come.
2 Corinthians 5:5 - Now it is God who has made us for this very purpose and has given us the Spirit as a deposit, guaranteeing what is to come.
Ephesians 1:13 - In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise, 14 who is the guarantee of our inheritance until the redemption of the purchased possession, to the praise of His glory.
Ephesians 4:30 - And do not grieve the Holy Spirit of God, by whom you were sealed unto/for the day of redemption.
4972. σφραγίζω sphragízō; fut. sphragísō, from sphragís (G4973), seal. To seal, trans.:
(I) To seal, close up and make fast with a seal signet such as letters or books so that they may not be read (Sept.: 1 Kgs. 21:8; Is. 29:11; Dan. 12:4). Hence, figuratively of lips, to keep in silence, not to make known, with the acc. (Rev. 10:4; 22:10; Sept.: Dan. 8:26). Generally, to seal or set a seal for the sake of security upon a sepulcher, prison, with the acc. (Matt. 27:66; Rev. 20:3; Sept.: Song 4:12). Figuratively, to secure to someone, make sure, deliver over safely, in the mid. with the acc. and dat. (Rom. 15:28 [cf. Deut. 32:34; 2 Kgs. 22:4]).
(II) Generally, to set a seal or mark upon a thing as a token of its authenticity or approvedness; used of persons, with the acc. (Rev. 7:3); pass. (Rev. 7:4-8). More often of decrees or documents, to attest by a seal (Sept.: Esth. 8:8, 10; Job 14:17). Hence figuratively, to attest, confirm, establish, with the acc. (John 6:27 [cf. 5:36]), followed by hóti (G3754), that (John 3:33). So also of Christians whom God attests and confirms by the gift of the Holy Spirit as the earnest, pledge, or seal of their election to salvation. Mid. with the acc. (2 Cor. 1:22); pass. (Eph. 1:13; 4:30).
σφραγίζω. 1aor. ἐσφράγισα, mid. ἐσφραγισάμην; pf. pass. ἐσφράγισμαι; 1aor. pass. ἐσφραγίσθην; seal; (1) literally seal up, secure by putting a seal on (MT 27.66); figuratively, as keeping something secret seal (up), conceal (RV 10.4); (2) as providing a sign of identification or ownership (mark with a) seal (RV 7.3); metaphorically, of endowment with the Spirit (EP 1.13); (3) figuratively, from the idea of an official seal on a document; (a) confirm, attest, certify (JN 3.33); (b) metaphorically, as a commercial technical term indicating a safely accomplished transaction σφραγίζειν τινί τὸν καρπὸν τοῦτον literally seal to someone this fruit, i.e. safely turn over to someone this kind provision (RO 15.28)
Then from AT Robertson...(esphragisthete). First aorist passive indicative of sphragizo, old verb, to set a seal on one as a mark or stamp, sometimes the marks of ownership or of worship of deities like stigmata (Ga 6:17). Marked and authenticated as God's heritage as in 4:30. See 2Co 1:22 for the very use of the metaphor here applied to the Holy Spirit even with the word arrabon (earnest). Spirit (pneumati). In the instrumental case.