en_tn_condensed/2co/01/22.md

he set his seal on us

"he has put his mark of ownership on us" or "he has shown that we belong to him"

gave us the Spirit in our hearts

"gave us the Spirit to live within each of us"

the Spirit ... as a guarantee

Paul says the Holy Spirit is the guarantee, which means pledge or down-payment, of a Christian's eternal life. Christians are securely saved. But they will not experience all of God's given promises until after they die. The Holy Spirit is a personal guarantee that this will happen.