21 It is God who enables us, along with you, to stand firm for Christ. He has commissioned us, 22 and he has identified us as his own by placing the Holy Spirit in our hearts as the first installment that guarantees everything he has promised us.
Paul mentions two gifts God gives when we become believers: (1) a seal of ownership to show who our Master is, and (2) the Holy Spirit, who guarantees that we belong to him and will receive all his benefits. The Holy Spirit guarantees that salvation is ours now, and that we will receive so much more when Christ returns. The great comfort and power the Holy Spirit gives in this life is a foretaste or down payment ("first installment") of the benefits of our eternal life in God's presence. With the privilege of belonging to God comes the responsibility of identifying ourselves as his faithful servants. Don't be ashamed to let others know that you are his.