Assurance comes from...
1. The Objectivity of the Story
2. The Promises of God
3. The Witness and Work of the Holy Spirit
The Objectivity of the Story1. "Salvation is not something we ourselves do, feel, think, wish or achieve. If it were, then we might rightly doubt whether we have done enough to be sure of being saved. Or we might worry that we don't feel saved. But the Bible makes it clear that salvation is something that God has accomplished. That means our salvation is grounded in the objectivity of the story. By objectivity, I mean something that stands outside and independent of ourselves...So assurance of salvation comes not from trusting in my own faith for its own sake, but rather from trusting in the object of my faith - what God has actually done to make my salvation possible."
The Promises of God2. Hebrews 6: 13-20 "For when God made a promise to Abraham, since he had no one greater by whom to swear, he swore by himself, saying, “Surely I will bless you and multiply you.” And thus Abraham, having patiently waited, obtained the promise. For people swear by something greater than themselves, and in all their disputes an oath is final for confirmation. So when God desired to show more convincingly to the heirs of the promise the unchangeable character of his purpose, he guaranteed it with an oath, so that by two unchangeable things, in which it is impossible for God to lie, we who have fled for refuge might have strong encouragement to hold fast to the hope set before us. We have this as a sure and steadfast anchor of the soul, a hope that enters into the inner place behind the curtain, where Jesus has gone as a forerunner on our behalf, having become a high priest forever after the order of Melchizedek."
"And you notice how the writer of Hebrews makes the example of Abraham relevant to us by saying at the end of the passage that we too can have the same rock-solid security in our faith because of the character and promise of God."
Romans 8: 28-30 "And we know that for those who love God all things work together for good, for those who are called according to his purpose. For those whom he foreknew he also predestined to be conformed to the image of his Son, in order that he might be the firstborn among many brothers. And those whom he predestined he also called, and those whom he called he also justified, and those whom he justified he also glorified."
The Witness and Work of the Holy Spirit3. "Is there not also a profoundly personal, internal, emotional and spiritual experience of salvation? Am I not actually supposed to feel saved? And of course the answer is a resounding and joyful 'YES'!...Then, once we have come into that new relationship with God through Christ, it is the Holy Spirit who bears witness within our own minds and hearts that it has indeed happened - that we have indeed been born again and have become children of God. God has made us sons and heirs, objectively. But it is the Spirit who gives us the inner assurance that it is indeed so, and our life of prayer, in which we call God 'Abba Father', as Jesus did, confirms that relationship."
Romans 8: 15-17 "For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of adoption as sons, by whom we cry, “Abba! Father!” The Spirit himself bears witness with our spirit that we are children of God, and if children, then heirs—heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him."
"So, another aspect of our assurance of salvation comes from the way our lives are being changed more and more into the image of Christ, through the work of the Holy Spirit. As we begin to bear the fruit of the Spirit, and to exercise the gifts of the Spirit, and to 'walk in the Spirit', then we grow in our assurance of salvation because the evidence of that salvation at work within us is being seen in our lives."