The most agonizing problem about the assurance of salvation is not the problem of whether the objective facts of Christianity are true (God exists, Christ is God, Christ died for sinners, Christ rose from the dead, Christ saves forever all who believe, etc.). Those facts are the utterly crucial bedrock of our faith. But the really agonizing problem of assurance is whether I personally am saved by those facts.
This boils down to whether I have saving faith. What makes this agonizing - for many in the history of the church and today - is that there are people who think they have saving faith but don't. For example, in Matthew 7:21-23, Jesus says, "Not everyone who says to Me, 'Lord, Lord,' will enter the kingdom of heaven, but he who does the will of My Father who is in heaven. Many will say to Me on that day, 'Lord, Lord, did we not prophesy in Your name, and in Your name cast out demons, and in Your name perform many miracles?' And then I will declare to them, 'I never knew you; depart from me, you who practice lawlessness.'"
So the agonizing question for some is: do I really have saving faith? Is my faith real? Am I self-deceived? Some well-intentioned people try to lessen the problem by making faith a mere decision to affirm certain truths, like the truth: Jesus is God, and he died for my sins. Some also try to assist assurance by denying that any kind of life-change is really necessary to demonstrate the reality of faith. So they find a way to make James 2:17 mean something other than what is seems to mean: "Even so faith, if it has no works, is dead." But these strategies to help assurance backfire. They deny some Scripture; and even the minimal faith they preserve can be agonized over and doubted by the tormented soul. They don't solve the problem, and they lose truth. And, perhaps worst of all, they sometimes give assurance to people who should not have it.
Instead of minimizing the miraculous, deep, transforming nature of faith, and instead of denying that there are necessary life-changes that show the reality of faith, we should tackle the problem of assurance another way. We should begin by realizing that there is an objective warrant for resting in God's forgiveness of my sins, and there is a subjective warrant for God's forgiveness of my sins. The objective warrant is the finished work of Christ on the cross that "has perfected for all time those who are being sanctified" (Hebrews 10:14). The subjective warrant is our faith which is expressed in "being sanctified."
Next we should realize that saving faith has two parts. First, faith is a spiritual sight of glory (or beauty) in the Christ of the gospel. In other words, when you hear or read what God has done for sinners in the cross and the resurrection of Jesus, this appears to your heart as a great and glorious thing in and of itself even before you are sure you are saved by it. I get this idea from 2 Corinthians 4:4, where Paul says that what Satan hinders in the minds of unbelievers is the "seeing the light of the gospel of the glory of Christ, who is the image of God." For faith to be real there must be a supernatural "light" that God shines into the heart to show us that Christ is glorious and wonderful (2 Corinthians 4:6). This happens as a work of the Spirit of God through the preaching of the gospel.
Second, faith is a warranted resting in this glorious gospel for our own salvation. I say "warranted resting" because there is an "unwarranted resting" - people who think they are saved who are not, because they have never come to see the glory of Christ as compellingly glorious. These people only believe on the basis of wanting rescue from harm, not because they see Christ as more beautiful and desirable than all else. But for those who "see the light of the gospel of the glory of Christ" their resting is warranted.
What this means practically is that we should continually look to the cross and the work of God in Christ, because this is where God makes the light of the gospel shine. Secondly, we should continually pray for God to "enlighten the eyes of our hearts" (Ephesians 1:18). Thirdly, we should love each other; because, as John said, "We know that we have passed out of death into life, because we love the brethren." In the end, assurance is a precious gift of God. Let us pray for each other that it will abound among us.
Seeing and resting,