Now he which stablisheth us with you in Christ, and hath anointed us, is God;
Stablisheth us with you in Christ - literally, into Christ; i:e., into ever-closer union with Christ.
Anointed us. As "Christ" is the "Anointed," so 'He hath anointed [Greek, chrisas, akin to Christos (G5547), Christianoi (G5546): aorist, once for all anointed - i:e., here in the general sense consecrated, called to the holy office of king and prophet to God (Revelation 1:6); but bebaioon (G950) is stablishing continually] us, alike ministers and believing people, with the Spirit (1 John 2:20-27). Hence, we become "a sweet savour of Christ" (2 Corinthians 2:15).
(Is) God; who hath also sealed - rather put the "is" after "God:" 'He which stablisheth us, and hath anointed us, namely, God, is He who also hath sealed us' - i:e., guaranteed the unchangeable truth of our preaching, by giving us His Holy Spirit, the crowning confirmation. A seal assures the possession of property to one; "sealed" is the crowning assurance of the Spirit (1 Corinthians 9:2).
The earnest of the Spirit - i:e., the Spirit as the earnest (i:e., money given by a purchaser as a pledge for the full payment of the sum premised). The Holy Spirit is to the believer now as a first instalment to assure him his full inheritance as a son of God shall be his hereafter (Ephesians 1:13-14; Romans 8:23): the pledge of the fulfillment of "all the promises" (2 Corinthians 1:20).