Christ himself declared that he was marked with his Father's seal. Christians are also marked with a seal: "It is God who establishes us with you in Christ and has commissioned us; he has put his seal on us and given us his Spirit in our hearts as a guarantee." This seal of the Holy Spirit marks our total belonging to Christ, our enrollment in his service for ever, as well as the promise of divine protection in the great eschatological trial.
Source: Catechism of the Catholic Church (Vatican.va)