In Him, you also, when you heard the word of truth, the good news of your salvation, and [as a result] believed in Him, were stamped with the seal of the promised Holy Spirit [the One promised by Christ] as owned and protected [by God]. – Ephesians 1:13 AMP
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 then, what is God saying to us when he gives us his Holy Spirit and calls him a guarantee or a down-payment?
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 forever. 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 forever and ever. And I tell you this here in Ephesians chapter 1 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.”
“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.”
– Desiring God