In Him, you also, after listening to the message of truth, the gospel of your salvation—having also believed, you were sealed in Him with the Holy Spirit of promise, who is given as a pledge of our inheritance, with a view to the redemption of God’s own possession, to the praise of His glory.
There are some circles of Christianity that believe salvation is something that can be lost. Growing up in a Southern Baptist home I was taught the doctrine often called “once saved always saved.” Having studied the mater as an adult, I believe Scripture teaches that none who are truly saved will fall away from the faith. Jesus’ parable of the sower indicated that many will hear the word and not believe, others would seem to have faith but would wither away or have their faith choked by the worries and temptations of this life, and still others would maintain and grow in their faith until the end. Although true believers may fall into sin or even periods of doubt, they are sealed in Christ by the Holy Spirit. Something within us tugs at our hearts and souls until we are back in a right relationship with God. Although we should daily ask God to create in us a clean heart and renew a right spirit within us, neglecting to confess a sin without receiving forgiveness of penance before we die will in no way cause us to lose our salvation.
"Sealed in Him" or “marked with a seal” indicates our security and God’s authority over the matter. God has labeled and claimed us as His children, and no power seen or unseen can negate that claim. We are under his divine protection no matter what life may hold. This is a pledge of our inheritance, a guarantee that we are redeemed and will one day stand sinless and forgiven before God, to the praise of His mercy and glory.
John Piper wrote, “God's great desire for his people is that we feel secure in his love and in his power. Everything else in life may be unstable -- our health, our family, our job, our education, our society, our world. At any of these levels you may feel as if you are out on a ledge forty stories up in an unpredictable wind. You feel yourself losing balance and falling, and every brick you grab pulls out of its mortar. So let's make sure we understand this from the outset: when you swore allegiance to Jesus Christ you signed up for the most dangerous mission in the world. How did Paul describe it? Tribulation, distress, persecution, famine, nakedness, peril, sword -- we are regarded as sheep to be slaughtered (Rom. 8:35-36)… God's desire for his people is that we feel secure in his love and in his power even if everything else in the world is uncertain…
“So then, what is God saying to us when he gives us his Holy Spirit and calls him a guarantee or a downpayment? He is saying, ‘My great desire for those who believe in me is that you feel secure in my love. I have chosen you before the foundation of the world. I have predestined you to be my children for ever. I have redeemed you by the blood of my Son. And I have put my Spirit in you as a seal and a guarantee. Therefore, you will receive the inheritance and praise the glory of my grace for ever and ever. And I tell you this here in Ephesians chapter one because I want you to feel secure in my love and my power. I don't promise you an easy life. In fact, through many tribulations you must enter the Kingdom (Acts 14:22). I don't promise always to speak in soft tones of approval, but to warn you in love whenever you begin to seek security in anything but me’ says the Lord.
“‘Let me say it again: I have chosen you,’ says the Lord. ‘I have predestined you; I have redeemed you; I have sealed you by my Spirit. Your inheritance is sure, because I am passionately committed to magnify the glory of my grace in your salvation.’”