There are at least two different ways we struggle with assurance, two different truths we want and need to be assured of. The first is the truth of the gospel. What if the Bible is just the opinions of men? What if Jesus is still dead? This struggle is real, but rather rare. The far more common struggle is over this truth- I am a child of God. Our doubts typically are less about Him, more about us. We know He is faithful and true. We doubt our faith is true.
One of the sources of that doubt, or perhaps better, that fear, is the devil himself. I would suggest that even when the devil is seeking to tempt you toward some sin the end goal isn’t merely that you’d commit the sin, but that having done so, he can use it to assail you as unworthy of God’s grace (which is true enough) and outside of God’s grace (which is a deep lie.) Second, he uses hardship in much the same way. He encourages us to believe that if we had God’s favor our lives would look a lot different. We would, if He loved us, enjoy greater wealth, greater health, greater everything. Our lack in these areas is his exhibit A that we are not under grace.
A second source of doubt is ourselves. And that can be a good thing. When we find ourselves buried deep in grievous sin, it’s a good thing to stop and consider, “Am I a child of God?” When David received the report that Uriah was dead his conscience should have assailed him. He was a murderer and an adulterer. His assurance should have been plummeting. It should not, however, stay there. Grievous sin, like all sin, should drive us toward Him, should lead us back to the cross, and to a deep confidence in His grace, including His grace toward us. Of course we’re not worthy of grace. No one is, or ever could be. A person worthy of grace is a square circle.
The third source I’d like to consider is mysterious. It is something I know precious little about. What I do know is that more often than not, when doubt is present, so is this- OCD. Obsessive Compulsive Disorder is a broad term describing that which drives some people to wash their hands over and over, others to never step on a crack in the sidewalk. The brain fixates on this or that, and is unable to escape. It can be the source of laughter, but it can also be debilitating. Where it comes from I don’t know. The cure for it I don’t know. I do know that I have had many conversations in which I am asked for counsel from someone who is afraid their faith isn’t deep enough, who fear they are not elect, who despair that they have committed the unpardonable sin. And when I ask them, “Have you ever been diagnosed with OCD?” they invariably reply, “How in the world did you know that?” Because the two, for whatever reason, tend to go together. And when they do, the sufferer doesn’t have an assurance problem, but an OCD problem.
God is always faithful. We never are. We are not repentant enough, trusting enough, obedient enough. Only Jesus is. If we rest in Him, we receive all that is His. One cannot “Rest harder.” One can only rest. We can all only cry out, “Lord be merciful to me, a sinner.” And then we can go home, knowing we have been justified.