"This endowment of the Spirit is the source of God's three closely connected actions within. The first-"God. . .christed us Christward"-must be regarded as deliberate play on words, signifying that God has "made us Christ's people," "a messianic community." Here Paul may be implicitly rebutting the alternative ministry of the Judaizers, as if to say that only through the Christ proclaimed by Paul (as opposed to the Jesus proclaimed by them) and the related experience of the Spirit have the Corinthians become Christ's people, the eschatological messianic community of the new covenant (3:3,6,8,18; cf. 11:2,4).
The second, "he set his seal of ownership on us," returns to and reinforces the commercial imagery associated with "he guarantees." The "seal"-often in wax-is a mark of ownership, but also a guarantee of authenticity. The "seal" is nothing less than the Spirit himself, by whom God has marked believers as his own ultimate possession. As in Eph. 1:13 and 4:30, the "seal" is here also eschatological. On the "day of redemption" God will take possession of those who are already sealed as his own." pg. 112 Paul Barnett