Carl F. Wisløff (1908-2004) was a Norwegian Lutheran theologian and preacher. I recently acquired a copy of his short systematic theology, I Know in Whom I Believe: Studies in Bible Doctrine (I ordered it from here).
Here is what Wisløff writes about assurance of salvation:
“In Romans 4:16, it is said of Abraham: “For this reason it is by faith, that it might be in accordance with grace, in order that the promise may be certain to all the descendants…”
“Here the divine purpose of justification by grace through faith is set forth. God saves us this way in order that we may be certain of our salvation. We read: “…to the end the promise might be sure.”
“He who would be justified by law, i.e., by his own pious deeds, can never be certain of his salvation. If some type of condition is set up for salvation, a term that must be fulfilled, than [sic] we can never be quite certain that we have fulfilled it. If it is said that we must love God above all else, then both the Law and our conscience testify that we have never loved God as we ought. If it is said that you must first have forgiven the one who sinned against you from the heart, you would never know whether you wholly have been equal of this forgiveness; one would again and again be anxious that there might be something that has not been forgiven. And if we say that the seeker must first surrender himself wholly to God before God can save him, then the poor creature would never surrender to God during this entire lifetime, because there would be one thing or other in thoughts, words and deed which is not entirely surrendered to Him. What sinful person can wholly surrender to God? He must be able to change his own nature, but that is just what he cannot do.
“The Roman Church clearly says that a person can never be certain that he is saved. This relates to her teaching regarding justification. The Roman Church does not believe that we are saved by faith alone; a man must at least have a beginning of love to God and a firm determination to flee sin. And who can be certain that he has reached this love and this firm determination? Nobody can.
“But he who believes the testimony of the Bible regarding justification can be certain in his case. For this Gospel places no conditions for salvation, it merely states that God is reconciled by virtue of Jesus’ finished work. Jesus has done all that must be done in order that we might be saved.
“The Gospel expressly places the basis of salvation outside of ourselves. We listen to the witness of the Word regarding the finished salvation, and while we do that the Spirit whispers in our heart: “It was for you.” Therefore, a Christian is certain that he possesses grace; he knows that as truly as God’s Word and promises are true, he is saved. The certainty of faith is a certainty regarding the truth of God’s Word” (pp. 85-86).
Wisløff also recognizes that Christians can begin to doubt their salvation due to sin, but there is a solution:
“We can come into doubt and anxiety, and the struggle can be difficult before we again can rest in God’s Word. Normally, a Christian is certain that he stands in a state of grace.
“…Certainly a believer can also get into uncertainty. This happens because he still has his evil flesh which will never believe God’s Word. The enemy of the soul also attempts to stimulate this uncertainty, in that he points to our sins and asks whether we think all this is becoming to a Christian. There is only one way out of such an uncertainty and that is to admit that it is true. In many ways we have acted contrary to God’s will and from our hearts we must pray for more power to do better in the future. Then we must cling to the Gospel, until the heart is quiet in the assurance that the blood of Jesus Christ, God’s Son, cleanses us from all sin” (pp. 86-87).