The Catechism in Questions & Answers

516. What does the term “sealing” / “sealed” mean in the letters of the New Testament?

In the letters of the New Testament, the term “sealing” / “sealed” refers to the imparting of the gift of the Holy Spirit: “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 Corinthians 1: 21-22).
“In Him [Christ] you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise” (Ephesians 1: 13).
“And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption” (Ephesians 4: 30).