And you also were included in Christ when you heard the message of truth, the gospel of your salvation. When you believed, you were marked in him with a seal, the promised Holy Spirit.
The Holy Spirit is like a seal, or a deposit, or an engagement ring: it is a way of God giving us a downpayment, which promises us that He will finish what He has started. When did you first hear the “message of truth, the gospel of your salvation”? How did you respond when you first heard it?