Eph 1:13 In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit,
The gospel is a dynamite. It blasts away selfishness, self righteousness, self complacency, self-delusion and sinful self reliance that we all struggle with. By the Gospel, the love of God is shed abroad in our hearts through the promised Holy Spirit (Rom 5:5).
The Holy Spirit’s task is to apply Christ’s work to God’s people. He seals and anoints believers “in Christ” the moment they believe. A seal in biblical times as today is used to guarantee security or indicate ownership.
The Holy Spirit validates/authenticates believers’ eternal life by sealing them because they trust in Christ, “4 For as many as are led by the Spirit of God, these are sons of God…6 The Spirit Himself bears witness with our spirit that we are children of God,“, Rom 8:4-6. The Spirit’s presence is God’s guarantee that believers are owned by Him and secured in Him. And now through the gospel, the Holy Spirit opens the souls of believers to a fountain of spiritual life that ought to impact them.