Jamieson, Fausset and Brown on 2 Corinthians 1:22

22. sealed--A seal is a token assuring the possession of property to one; "sealed" here answers to "stablisheth us" (2Co 1:21; 1Co 9:2).

the earnest of the Spirit--that is, the Spirit as the earnest (that is, money given by a purchaser as a pledge for the full payment of the sum promised). The Holy Spirit is given to the believer now as a first instalment to assure him his full inheritance as a son of God shall be his hereafter (Ep 1:13, 14). "Sealed with that Holy Spirit of promise which is the earnest of our inheritance until the redemption of the purchased possession" (Ro 8:23). The Spirit is the pledge of the fulfilment of "all the promises" (2Co 1:20).