“Now He who establishes us with you in Christ and has anointed us is God, who also has sealed us and given us the Spirit in our hearts as a guarantee.” (2 Cor 1:21-22)
The promised Holy Spirit is a seal. God gives His Holy Spirit to His believers as a guarantee of our inheritance from heaven. The fact that God has chosen us to believe in the Lord Jesus Christ and poured out His promised Holy Spirit on us as a seal and guarantee is a powerful testimony that our hope in Christ is sure.